pH-sensitive hydrogels composed of poly( vinyl pyrrolidone)/acrylic acid (PVP/AAc) were prepared by means of gamma-radiation induced graft copolymerization and crosslinking. The effects of pH and ionic strength on the equilibrium of swelling for the prepared hydrogels were investigated. Studying the diffusion's parameters and coefficient as a function of the pH values for the surrounding solution showed drastic changes in the swelling parameters as a result of changing the surrounding solution pH values. To estimate the ability of the prepared copolymer to be used as a colon-specific drug carrier, the release of indomethacin was monitored as a function of time at pH 1 and 7. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 3484-3489, 2011