In this study, first polyacrylamide hydrogels were synthesized by free-radical crosslinking polymerization of acrylamide monomer with N,N'-methylenebis(acrylamide) as a crosslinker in an aqueous solution at 22 degrees C. Then, a series of hydrogels at various charge densities were prepared by partial hydrolysis of polyacrylamide precursors in a 0.1M sodium hydroxide solution at 60 degrees C. The hydrolysis time was varied between 20 and 180 min. The chemical structures and internal morphologies of the hydrogels before and after alkaline hydrolysis were characterized with attenuated total reflectance/ Fourier transform infrared and scanning electron microscopy measurements. The swelling ratio of the hydrolyzed hydrogels was measured in buffer solutions at various pHs. From differential curves of dQ(v)/dpH versus pH (where Q(v) is the equilibrium swelling ratio of the hydrogels), the volume phase transition pH of the hydrogels was found to be 4.33 +/- 0.05, regardless of the charge density of the hydrogels. In water at 22 degrees C, the hydrogels with greater charge density showed a more rapid swelling rate because of their higher porosity and hydrophilicity. Moreover, in buffer solutions with the pH changing from 9.0 to 2.0 at 22 degrees C, the hydrogels with greater charge density also exhibited a more rapid deswelling rate than the hydrogels with less charge density. In conclusion, the postmodification method is a good way of preparing pH-sensitive hydrogels with fast responsiveness. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 108-113, 2009