Exhibit 3.1

Received at: 1:31PM, 5/14/2008

05/14/2008 1:29:42 PM FAXCOM PAGE 2 OF 5

Michigan Department of Labor & Economic Growth

Filing Endorsement

This is to Certify that the CERTIFICATE OF AMENDMENT — CORPORATION

for ISABELLA BANK CORPORATION

ID NUMBER: 346543

received by facsimile transmission on May 14, 2008 is hereby endorsed Filed on May 14, 2008 by the Administrator.

The document is effective on the date filed, unless a subsequent effective date within 90 days after received date is stated in the document.

In testimony whereof, I have hereunto set my

hand and affixed the Seal of the Department, in the City of Lansing, this 14TH day of May, 2008.

Director

Bureau of Commercial Services

Sent by Facsimile Transmission 08135

BC8/CD-51S (Rev. 3/07) — MICHIGAN DEPARTMENT OF LABOR & ECONOMIC GROWTH BUREAU OF COMMERCIAL SERVICES — Date Received (FOR BUREAU USE ONLY)

This document is effective on the date filed, unless a subsequent effective data within 90 days after received date Is stated In the document. —— — Name Matt G. Hrebec —— —— — Address 313 South Washington Square —— —— — City            State            Zip Code Lansing            Ml 48933-2193 [effective date: —— —— —

Document will be returned to the name and address you enter above. If left blank document will be mailed to the registered office.

CERTIFICATE OF AMENDMENT TO THE ARTICLES OF INCORPORATION

For use by Domestic Profit and Nonprofit Corporations (Please read information and instructions on the last page)

Pursuant to the provisions of Act 284, Public Acts of 1972 (profit corporations), or Act 162, Public Acts of 1982 (nonprofit corporations), the undersigned corporation executes the following Certificate:

1. The present name of the corporation is: IBT Bancorp, Inc.

2. The identification number assigned by the Bureau is: 346-543

3. Article I, II and III            of the Articles of Incorporation is hereby amended to read as follows:

See attached Exhibit A.

1.

COMPLETE ONLY ONE OF THE FOLLOWING:

4. Profit or Nonprofit Corporation: For amendments adopted by unanimous consent of incorporators before

the first meeting of the board of directors or trustees.

The foregoing amendment to the Articles of Incorporation was duly adopted on the ......... day of         , in accordance with the provisions of the Act by the unanimous consent of the incorporator(s) before the first meeting of the Board of Directors or Trustees. Signed this ......................... day of

(signature)___(Signature)

(Type or Print Name) (Type or Print Name)

(Signature) (Signature) (Type or Print Name) (Type or Print Name)

5. Profit Corporations Only: Shareholder or Board Approval

The foregoing amendment to the Articles of Incorporation was duly adopted on the 13^th day of May ....................... , 2008 .................... by the: (check one of the following) —— —

shareholders at a meeting in accordance with Section 611 (3) of the Act.

written consent of the shareholders having not less than the minimum number of votes required by statute in accordance with Section 407(1) of the Act, Written notice to shareholders who have not consented in writing has been given. (Note: Written consent by less than all of the shareholders is permitted only if such provision appears in the Articles of Incorporation.)

written consent of all the shareholders entitled to vote in accordance with section 407(2) of the Act. board of a profit corporation pursuant to section 611(2) of the Act.

Profit Corporations and Professional Service Corporations Signed this 14^th day of .. May ............... ,2008

—— —— —

(Signature of an authorized officer or agent)

Dennis P. Angner, President

(Type or Print Name)

Exhibit A Certificate of Amendment to the Articles of Incorporation

Of IBT Bancorp, Inc.

3. Article I, II and III of the Articles of Incorporation are hereby amended to read as follows:

Article I

The name of the corporation is Isabella Bank Corporation.

Article II

The purpose or purposes for which the corporation is organized is to be registered as a bank holding company under the Bank Holding Company Act, being 12 U.S.C. Sections 1841 to 1850.

Article III

The total authorized capital stock of the Corporation is Fifteen Million (15,000,000) shares of common stock with no par value, all of one class with voting rights.

BCS/CD-515 (Rev. 3/07) Name of person or organization remitting fees: Preparer’s name and business telephone number Foster, Swtfl, Collins & Smith, P.C. ......... Matt G, Hrebec —— — (517 ) 371-8256 —

INFORMATION AND INSTRUCTIONS

1. This form may be used to draft your Certificate of Amendment to the Articles of Incorporation, A document required or permitted to be filed under

the act cannot be filed unless it contains the minimum information required by the act. The format provided contains only the minimal information
required to make the document fileable and may not meet your needs. This is a legal document and agency staff cannot provide legal advice.

2. Submit one original of this document. Upon filing, the document will be added to the records of the Bureau of Commercial Services. The original

will be returned to your registered office address, unless you enter a different address in the box on the front of this document.

Since this document will be maintained on electronic format. It is important that the filing be legible. Documents with poor black and white contrast, or otherwise illegible, will be rejected.

3. This Certificate is to be used pursuant to the provisions of section 631 of Act 284, P.A. of 1972 or Act 162, P.A. of 1982, for the purpose of

amending the Articles of Incorporation of a domestic profit corporation or nonprofit corporation. Do not use this form for restated articles.

4. Item 2 — Enter the identification number previously assigned by the Bureau. If this number Is unknown, leave it blank.

5. Item 3 — The article(s) being amended must be set forth in its entirety. However, if the article being amended is divided into separately Identifiable

sections, only the sections being amended need be included.

6. If the amendment changes the term of existence to other than perpetual, all nonprofit corporations except churches must obtain a consent to

dissolution, or a written statement that the consent is not required, from the Michigan Attorney General, Consumer Protection and Charitable Trusts Division, P.O. Box 30214, Lansing, Ml 48909, (517)373-1152. Application for the consent should be made at least 45 day before the desired effective date of the dissolution. This certificate cannot be filed unless It is accompanied by the consent or written statement

7. This document is effective on the date endorsed “filed” by the Bureau. A later effective date, no more than 90 days after the date of delivery, may

be stated as an additional article.

8. Signatures:

Profit Corporations: (Complete either Item 4 or Item 5)

1) Item 4 must be signed by at least a majority of the Incorporators listed in the Articles of Incorporation.
2) Item 5 must be signed by an authorized officer or agent of the corporation.

Nonprofit Corporations: (Complete either Item 4 or Item 6)

1) Item 4 must be signed by all of the Incorporators listed in the Articles of Incorporation.
2) Item 6 must be signed by either the president, vice-president, chairperson or vice-chairperson.

9. FEES: Make remittance payable to the State of Michigan. Include corporation name and identification number on check or money order.

NONREFUNDABLE FEE: $10.00 ADDITIONAL FEES DUE FOR INCREASED AUTHORIZED SHARES OF PROFIT CORPORATIONS ARE: Amount of IncreaseFee —

1-60,000$50.00 60,001-1,000,000$100 1,000,001-5,000,000$300 5,000,001-10,000,000$500 More than 10,000,000$500 for first 10,000,000 plus $1000.00 for each additional 10,000,000, or portion thereof. To submit by mail: To submit In person:

Michigan Department of Labor & Economic Growth 2501 Woodlake Circle Bureau of Commercial Services — Corporation Division            Okemos, Ml P.O. Box 30054 Telephone: (517) 241 -6470

Lansing, Michigan 48909

Fees may be paid by VISA or Mastercard when delivered in
person to our office.

MICH-ELF (Michigan Electronic Filing System): First time users: Call (517) 241-6470, or visit our website at http://www.michigan.gov/corporations Customer with MICH-ELF Fiter Account: Send document to (517) 636-6437

The Department of Labor & Economic Growth will not discriminate against any individual or group because of race sex, religion, age, national origin, color, marital status, disability or political beliefs. If you need help with reading, writing, hearing, etc., under the Americans with Disabilities Act, you may make your needs known to this agency.

New expedited services beginning January 1,2006.

Expedited review and filing, if fileable, is available for all documents for profit corporations, limited liability companies, limited partnerships and nonprofit corporations.

The expedited service fees are in addition to the regular fees applicable to the specific document:

Please complete a separate BCS/CD 272 form for expedited service for each document via in person, mail and MICH-ELF.

24-hour service — $50 for formation documents and applications for certificate of authority.

24-hour service — $100 for any document concerning an existing entity.

Same day service

• Same day-$100 for formation documents and applications for certificate of

authority.

Same day — $200 for any document concerning an existing entity

Review completed on day of receipt. Document and request for same day expedited service must be received by 1 p.m. EST or EDT.

• Two hour — $500

Review completed within two hours on day of receipt. Document and request for two hour expedited service must be received by 3 p.m. EST or EDT.

• One hour-$1000

Review completed within one hour on day of receipt. Document and request for one hour expedited service must be received by 4 p.m. EST or EDT.

First time MICH-ELF user requesting expedited service must obtain a MICH-ELF filer number prior to submitting a document for expedited service. BCS/CD-901

Changes to information on MICH-ELF user’s account must be submitted before requesting expedited service, form BCS/CD-901.

Rev. 8/06