EXHIBIT NO. 99.(a) 31

MFS SERIES TRUST X

CERTIFICATION OF AMENDMENT

TO THE DECLARATION OF TRUST

REDESIGNATION OF CLASS R5 SHARES

Pursuant to Sections 6.10 and 9.3 of the Amended and Restated Declaration of Trust dated December 16, 2004, as amended (the “Declaration”), of MFS Series Trust X, a business trust organized under the laws of The Commonwealth of Massachusetts (the “Trust”), the undersigned Trustees of the Trust, being a majority of the Trustees of the Trust, do hereby redesignate all existing Class R5 Shares (as defined in the Declaration) of MFS Absolute Return Fund, MFS Aggressive Growth Allocation Fund, MFS Blended Research Growth Equity Fund, MFS Blended Research Mid Cap Equity Fund,  MFS Blended Research Small Cap Equity Fund, MFS Blended Research Value Equity Fund, MFS Conservative Allocation Fund, MFS Emerging Markets Debt Fund, MFS Emerging Markets Debt Local Currency Fund, MFS Emerging Markets Equity Fund, MFS Global Bond Fund, MFS Growth Allocation Fund, MFS International Diversification Fund, MFS International Growth Fund, MFS International Value Fund, MFS Managed Wealth Fund, and MFS Moderate Allocation Fund, each a series of the Trust, as “Class R6 Shares”.

Pursuant to Section 10.1 of the Declaration, this redesignation shall be effective on August 26, 2016.

IN WITNESS WHEREOF, a majority of the Trustees of the Trust have executed this amendment, in one or more counterparts, all constituting a single instrument, as an instrument under seal in The Commonwealth of Massachusetts, as of August 12, 2016, and further certify, as provided by the provisions of Section 9.3(c) of the Declaration, that this amendment was duly adopted by the undersigned in accordance with Section 9.3(a) of the Declaration.

STEVEN E. BULLER	JOHN P. KAVANAUGH
Steven E. Buller	John P. Kavanaugh
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
ROBERT E. BUTLER	ROBERT J. MANNING
Robert E. Butler	Robert J. Manning
c/o MFS Investment Management	MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
MAUREEN R. GOLDFARB	MARYANNE L. ROEPKE
Maureen R. Goldfarb	Maryanne L. Roepke
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
DAVID H. GUNNING	ROBIN A. STELMACH
David H. Gunning	Robin A. Stelmach
c/o MFS Investment Management	MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199
WILLIAM R. GUTOW	LAURIE J. THOMSEN
William R. Gutow	Laurie J. Thomsen
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199

MICHAEL HEGARTY	ROBERT W. UEK
Michael Hegarty	Robert W. Uek
c/o MFS Investment Management	c/o MFS Investment Management
111 Huntington Avenue	111 Huntington Avenue
Boston, MA 02199	Boston, MA 02199