Modification reactions are reported for fully-hydrogenated butadiene-acrylonitle (35.8 mol-% AN) copolymer, ethylene(E)-butyl acrylate (4.7 mol-% BA) copolymer, and ethylene(E)-methyl acrylate (45 mol-% MA) copolymer in dense, near-critical water at 250 "C and 300 "C and at 300 bar and 1 500 bar. Nitrile, amide, and ester moieties can be converted into COOH groups. Kinetic analysis of the ester to acid transformations suggests autocatalytic activity of the acid groups.