The photoinduced graft polymerization of acrylic acid and n-butyl acrylate on polyvinyl chloride or carbon monoxide-vinyl chloride copolymer by using benzophenone as a sensitizer was carried out. In graft polymerization of acrylic acid, the degree of grafting was low. However, graft polymerization of n-butyl acrylate or acrylic acid-n-butyl acrylate were higher than that of acrylic acid. Turbidimetry, DTA, and TGA of the graft polymer were examined. As the graft polymer was a weak polyacid, the pKawas estimated according to the Henderson equation, and the relationship between pKaand the acrylic acid unit content in the graft polymer was derived.