The effects of continuous water exchange on the swelling behavior of poly(N-isopropylacrylamide-co-sodium acrylate) gel were studied. The diameter of gels and the Na+ concentration in the solvent were measured at several constant intervals after the solvent (the distilled deionized water) was exchanged. The diameter decreased at room temperature as the solvent was exchanged with water, and it continued to decrease even after more than about 97% of the initial Na+ diffused into the water. Thus, the final swelling ratio of the gel was only slightly larger than that of the neutral poly(N-isopropylacrylamide) gel. To reveal the structural change in molecular level, solid-state C-13 cross-polarization/magic-angle spinning, solid-state H-1 combined rotation and multiple-pulse spectroscopy, and swollen-state C-13 dipolar decoupled/magic-angle spinning NMR experiments were carried out for several dried and swollen samples by varying the times of water exchange. As a result, the intensity and position of the carboxyl peak changed, and the relative intensity of the nonionized carboxyl groups of gels increased with an increasing number of water exchanges. These results indicated that hydrogen bonding was formed between the two, nonionized carboxyl groups (-COOH) and/or between the -COOH and -CONH- groups. The macroscopic polymer network shrinkage is discussed in terms of the replacement of counterions Na+ by H+ and of the formation of intermolecular hydrogen bonding. (C) 2004 Wiley Periodicals, Inc.