The radical copolymerization of 1-acryloyl-and 1-methacryloyl-3-methylpyrazole (1 a and l b ) with acrylic acid or methacrylic acid in solution was studied. Copolymers containing carboxyl as well as 3-methyl-1-pyrazolylcarbonyl groups as side chains were prepared via the reaction of 3(5)-methylpyrazole with activated poly(acry1ic acid)s of various molecular weights.The hydrolysis data suggest that the release rates of the biocide 3(5)-methylpyrazole from the investigated copolymers vary significantly depending on the content of carboxyl groups and their change during the hydrolysis.