Abstract: A carboxyl-containing ethylene copolymer composition exhibiting thermal stability and method for preparing the same. The composition comprises (a) an ethylene copolymer containing a carboxyl-functional comonomer in an amount ranging from about 0.01 to about 50 weight percent of the copolymer, and (b) a hydrated compound in intimate admixture therewith. The hydrate dehydrates at a temperature between about 100.degree. C. and about 300.degree. C. and is selected from hydrated metallic salts of organic acids, inorganic hydrates, bimetallic hydrates and combinations thereof. The hydrate is present in the composition in at least an amount effective to impart thermal stability thereto up to about 5 weight percent of the composition, preferably 0.01-0.45 weight percent. The composition is prepared by adding the hydrate to the copolymer and intimately mixing the hydrate and the copolymer. The addition and mixing steps are performed either during or subsequent to the manufacture of the copolymer.