We studied the ionization equilibria of the COOH groups bound to the copolymer and to the cross-linked copolymer network (i.e., gel) consisting of N-isopropylacrylamide (NIPA) and acrylic acid (AAc) by means of potentiometric titration with NaOH. Microgel particles (average hydrodynamic diameter similar to 120 nm in the acid form and at 25 degrees C; COOH content similar to 30 mol %) were employed, in addition to the usual bulk gel (of a fine grind) with the same COOH content. The microgel was prepared by aqueous redox polymerization using N,N＇-methylenebis(acrylamide) as a cross-linker and sodium dodecylbenzenesulfonate as a surfactant. The titration was carried out at 25 and 35 degrees C in the absence and the presence of NaCl. The dependence of the apparent dissociation constant (pK(a)) on the degree of dissociation (alpha(d)) was investigated. It was found that, on the whole, pK(a) increased with increasing alpha(d) but decreased by addition of NaCl. Over a wide ad range there was little difference in pK(a) between both gels, as well as between the gel and copolymer. At 35 degrees C and at alpha(d) < 0.15, however, an increase in ad brought about a distinct fall in pK(a) for the microgel as well as the copolymer. At alpha(d) < 0.15, pK,was larger at 35 degrees C than at 25 degrees C. To discuss these results in connection with the swelling behavior of the gel, size changes of the microgel particles were studied using the photon correlation spectroscopy. The size of the microgel increased with increasing the degree of neutralization (alpha(n)) but decreased by screening the COO- charges with counterions. Over the alpha(n) range of 0.6 to 1.0, there was little or no increase in the particle size. These aspects were analogous to those generally observed in the chain expansion of poly(AAc) in solutions, suggesting that the "counterion binding effect" plays an important role in the swelling of NIPA/AAc gels at high charge densities. It was also suggestive that hydrogen bonding as well as hydrophobic interaction affects the ionization of COOH and thereby the swelling degree at 35 degrees C and at alpha(d) < 0.15. Therefore, it may be concluded that our ionic NIPA gels swell through an increase in the net charge density due to the COO- ions rather than a rise in the osmotic pressure arising from mobile counterions within the gel phase.