A temperature-induced volume change of ionic gels was investigated for cross-linked copolymer gels of N-isopropylacrylamide and sodium acrylate (NIPA-AA) at various pH values, The volume change of the gel against temperature was discontinuous below pH 6.3, but it was continuous above pH 7.5. The continuous volume change observed for the completely ionized NIPA-AA gel suggests that an increase in the counterion osmotic pressure alters the discontinuous volume change of NIPA gel to the continuous one. Potentiometric titrations revealed that the dissociation constant of carboxyl groups of the linear copolymer of N-isopropylacrylamide and acrylic acid in solution decreased with increased temperature above 34 degrees C. The similar decrease in the carboxyl ionization with increasing temperature was also observed for NIPA-AA gel. The discontinuous volume change was correlated with the two effects arising from the decrease in the carboxyl ionization of NIPA-AA gel : the reduced Donnan osmotic pressure and the presence of a significant fraction of un-ionized carboxyl groups in the gel at the collapsed state.