Abstract: Process for preparing hydrolyzed ethylene-vinyl acetate copolymers, whose ethylene content is 20 to 50% by mole and degree of hydrolysis in vinyl acetate units is more than 90% by mole, having improved heat stability, which comprises, after hydrolyzing ethylene-vinyl acetate copolymer in an alcoholic or water medium in the presence of alkaline catalyst, contacting particles of the hydrolyzed copolymer with phosphoric acid such as orthophosphoric acid, phosphorous acid and pyrophosphoric acid to incorporate the phosphoric acid into the hydrolyzed copolymer in an amount of 0.001 to 0.5% by weight of phosphate radical calculated as --PO.sub.4 radical, to the dry weight of the hydrolyzed copolymer, and of not less than equal amount by weight to alkali metal ion remaining in the hydrolyzed copolymer.