Hydroxy radical-initiated poly(methyl methacrylate) and polystyrene have been reacted with o-sulfobenzoic anhydride to produce dye-active sulfonate groups and these were measured by a dye-partition technique with methylene blue. The important advantages over chlorosulfonic acid, previously employed in the dye-partition analysis for the conversion of hydroxy into sulfate groups, are that o-sulfobenzoic anhydride does not react at sites other than the hydroxy functionality and that it introduces the dye-active moiety (sulfonate) into the polymer via a hydrolytically more stable linkage.