0000063075 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006724Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006726Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006727Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006728Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006730Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006732Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006733Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000006725Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:C000126364Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:PrincipalRisksMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member rr:RiskLoseMoneyMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member rr:RiskNotInsuredMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:InvestmentSelectionRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:DebtMarketRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:InterestRateRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:CreditRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:Inflation-AdjustedDebtInstrumentsRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:ForeignRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:FocusRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:PrepaymentExtensionRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:DerivativesRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:LeveragingRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:CounterpartyandThirdPartyRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:LiquidityRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:LargeShareholderRiskMember 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member rr:AfterTaxesOnDistributionsMember cik0000063075:C000006724Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member rr:AfterTaxesOnDistributionsAndSalesMember cik0000063075:C000006724Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:BloombergUSAggregateBondIndex1Member 2024-10-31 2024-10-31 0000063075 cik0000063075:S000002493Member cik0000063075:BloombergUSTreasuryInflationProtectedSecuritiesIndex2Member 2024-10-31 2024-10-31 iso4217:USD pure utr:Y N-1A MFS® SERIES TRUST IX 2025-02-28 2025-02-28 MFS® Inflation-Adjusted Bond Fund Summary of Key Information MIAAX MIABX MIACX MIAIX MIALX MIATX MIAHX MIAJX MIAKX Investment Objective The fund’s investment objective is to seek total return that exceeds the rate of inflation over the long-term, with an emphasis on current income, but also considering capital appreciation. Fees and Expenses This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. More information about these and other waivers and reductions is available from your financial intermediary and in “Sales Charges and Waivers and Reductions” on page 11 and “Appendix A – Waivers and Reductions of Sales Charges” on page A-1 of the fund’s prospectus. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. 100000 Shareholder Fees (fees paid directly from your investment): 0.0425 0 0 0 0 0 0 0 0 0.0100 0.0400 0.0100 0 0 0 0 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0025 0.0100 0.0100 0 0.0100 0.0050 0.0025 0 0 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0004 0.0082 0.0157 0.0157 0.0057 0.0157 0.0107 0.0082 0.0057 0.0049 -0.0018 -0.0018 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 0.0064 0.0139 0.0149 0.0049 0.0149 0.0099 0.0074 0.0049 0.0041 Example This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods (unless otherwise indicated); your investment has a 5% return each year; and the fund’s operating expenses remain the same. Although your actual costs will likely be higher or lower, under these assumptions your costs would be: 488 658 844 1379 542 778 1038 1650 142 478 838 1650 252 488 848 1659 152 488 848 1659 50 175 310 706 152 488 848 1861 101 332 582 1298 76 254 447 1006 50 175 310 706 42 149 266 608 Portfolio Turnover The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 27% of the average value of its portfolio. 0.27 Principal Investment Strategies MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests at least 80% of the fund’s net assets in inflation-adjusted debt instruments and other instruments with similar economic characteristics. MFS generally focuses the fund’s investments in inflation-adjusted debt instruments issued by the U.S. Treasury. MFS may also invest the fund’s assets in other inflation-adjusted debt instruments and non-inflation-adjusted debt instruments, including corporate bonds, foreign government securities, securitized instruments, and other debt instruments.MFS generally invests substantially all of the fund’s assets in investment grade quality debt instruments.MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.MFS may invest the fund’s assets in foreign securities. While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate or currency exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, and swaps.MFS allocates the fund’s assets across maturities and types of debt instruments based on its evaluation of macroeconomic factors, including interest rates, inflation rates, and monetary and fiscal policies.MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative screening tools that systematically evaluate instruments may also be considered. In structuring the fund, MFS also considers top-down factors.For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes. Principal Risks As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.The principal risks of investing in the fund are: Principal Risks As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results. Debt Market Risk: Debt markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single instrument, issuer, or borrower, a particular type of instrument, issuer, or borrower, a segment of the debt markets or the debt markets generally. Certain events can have a dramatic adverse effect on debt markets and may lead to periods of high volatility and reduced liquidity in a debt market or segment of a debt market. Interest Rate Risk: In general, the price of a debt instrument falls when interest rates rise and rises when interest rates fall. Interest rate risk is generally greater for instruments with longer maturities or durations, or that do not pay current interest. Credit Risk: The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in, or perceptions of, the financial condition of the issuer, borrower, counterparty, or other entity, or underlying collateral or assets, or changes in, or perceptions of, specific or general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. Debt instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could negatively affect the market value and liquidity of a debt instrument.U.S. Government securities not supported as to the payment of principal or interest by the U.S. Treasury are subject to greater credit risk than are U.S. Government securities supported by the U.S. Treasury. Inflation-Adjusted Debt Instruments Risk: Interest payments on inflation-adjusted debt instruments can be unpredictable and vary based on the level of inflation. If inflation is negative, principal and income can both decline. Foreign Risk: Exposure to foreign markets through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These factors can make foreign investments, especially those tied economically to countries with developing economies or countries subject to sanctions or the threat of new or modified sanctions, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Focus Risk: Issuers in a single country or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, public health, and other conditions, and the fund's performance will be affected by the conditions in the countries and regions to which the fund is exposed. If MFS invests a significant percentage of the fund's assets in a single issuer or small number of issuers, the fund’s performance could be more volatile than the performance of more diversified funds. Prepayment/Extension Risk: Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended. Derivatives Risk: Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost. Derivatives can involve leverage. Leveraging Risk: Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses. Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability or willingness to perform in accordance with the terms of the transaction. Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs. Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would. Performance Information The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606. The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests. The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. mfs.com 1-800-225-2606 Class A Bar Chart. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. During the period(s) shown in the bar chart, the highest quarterly return was 4.94% (for the calendar quarter ended December 31, 2023) and the lowest quarterly return was (5.98)% (for the calendar quarter ended June 30, 2022). highest quarterly return 0.0494 2023-12-31 lowest quarterly return -0.0598 2022-06-30 Performance Table. Average Annual Total Returns (For the Periods Ended December 31, 2024) Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes -0.0325 -0.0007 0.0087 -0.0046 0.0015 0.0077 0.0166 0.0115 0.0160 0.0045 0.0014 0.0060 0.0105 0.0066 0.0112 0.0129 0.0092 0.0137 0.0155 0.0116 0.0161 0.0163 0.0125 0.0170 -0.0292 0.0013 0.0103 Returns After Taxes on Distributions -0.0442 -0.0159 -0.0021 Returns After Taxes on Distributions and Sale of Fund Shares -0.0172 -0.0059 0.0027 Reflects no deduction for fees, expenses, or taxes Reflects no deduction for fees, expenses, or taxes Bloomberg U.S. Aggregate Bond Index 0.0125 -0.0033 0.0135 Bloomberg U.S. Treasury Inflation Protected Securities Index 0.0184 0.0187 0.0224 Effective May 1, 2024, the Bloomberg U.S. Aggregate Bond Index replaced the Bloomberg U.S. Treasury Inflation Protected Securities Index as a broad measure of market performance in accordance with new regulatory disclosure requirements. The fund continues to use the Bloomberg U.S. Treasury Inflation Protected Securities Index as an additional benchmark that MFS believes more closely reflects the market segments in which the fund invests.After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. Effective May 1, 2024, the Bloomberg U.S. Aggregate Bond Index replaced the Bloomberg U.S. Treasury Inflation Protected Securities Index as a broad measure of market performance in accordance with new regulatory disclosure requirements. The fund continues to use the Bloomberg U.S. Treasury Inflation Protected Securities Index as an additional benchmark that MFS believes more closely reflects the market segments in which the fund invests. After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. 485BPOS 2025-02-28 2024-10-31 0000063075 2025-02-27 false ~ http://www.MFS.com/20241031/role/RRSchedule4 ~ ~ http://www.MFS.com/20241031/role/RRSchedule5 ~ ~ http://www.MFS.com/20241031/role/RRSchedule6 ~ ~ http://www.MFS.com/20241031/role/RRSchedule7 ~ ~ http://www.MFS.com/20241031/role/RRBarChart8 ~ ~ http://www.MFS.com/20241031/role/RRSchedule9 ~ -0.0235 0.0398 0.0241 -0.0197 0.0799 0.1025 0.045 -0.1191 0.0214 0.0139 This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. February 28, 2026 February 28, 2026 February 28, 2026 February 28, 2026 February 28, 2026 February 28, 2026 February 28, 2026 February 28, 2026 February 28, 2026 ~ http://www.MFS.com/20241031/role/RRSchedule10 ~ This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. Massachusetts Financial Services Company (MFS) has agreed in writing to reduce its management fee to 0.40% of the fund's average daily net assets annually up to $1 billion. This written agreement will remain in effect until modified by the fund's Board of Trustees, but such agreement will continue until at least February 28, 2026. MFS has agreed in writing to waive at least 0.01% of the fund's management fee as part of an agreement pursuant to which MFS has agreed to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund's Board of Trustees. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least February 28, 2026. MFS Fund Distributors, Inc. (MFD), has agreed in writing to waive the Class A and Class B service fee to 0.15% of each class' average daily net assets annually until modified by the fund's Board of Trustees, but such agreement will continue until at least February 28, 2026. Shares automatically convert to Class A shares approximately eight years after purchase; therefore, the expense examples reflect Class A share expenses after eight years.