The generation of carbonyl groups during sulfonation of porous styrene-divinylbenzene copolymers is reported. The carbonyl groups and sulfonic acid groups were verified by infrared spectroscopy and quantified by acid-base titrations. A gradual change in sulfonation reaction conditions from 70 degrees C/30 min to 98 degrees C/60 min resulted in a gradual increase in sulfonic acid group concentration from similar to 1.85 to similar to 3.13 mol/kg and in the carboxylic acid group concentration from similar to 0 to similar to 0.34 mol/kg. Data on the swelling coefficients in water indicate a slight increase in cross-linking during sulfonation, Conversion of ethyl or residual vinyl groups to carboxylic acid groups, followed by conversion of some of the acid groups to cross-links through an acid-catalyzed acylation reaction, is proposed. The proposed reactions also explain the pH-dependent color change of the resins. Sulfonation of copolymers with varied cross-linkages further supports our assertion that, as the concentrations of ethyl and residual vinyl groups increase, the concentration of -COOH groups also increases and the resin color becomes darker.