Hydrogels composed of N,V-dimethylacrylamide and a sulfonamide monomer (SDM, pK(a) = 6.17) derived from sulfadimethoxine exhibited a pH-induced volume-phase transition in water. This resulted from pH-dependent ionization/deionization of the sulfonamide group and reversible side group crystallization (physical cross-linking). The pH-dependent equilibrium swelling pattern was controlled by the polymer composition. At higher SDM content, a higher transition pH and a larger and sharper transition were observed, leading to almost discontinuous swelling transition of a gel containing 40 mol % SDM around the physiological pH. It is evident that side group crystallization was primarily responsible for such a sharp transition, influenced by temperature-induced chain flexibility and associated with hydrogen bonding between un-ionized sulfonamide groups. However, after crystal melting at 70 degreesC, the pH dependency in swelling was well-preserved but with reduced sharpness, suggesting a partial role of hydrophobic interaction in the pH-induced volume-phase transition.