Interpenetrating polymer network (IPN) hydrogels based on poly(vinyl alcohol) and 1-vinyl-2-pyrrolidone were prepared by radical polymerization with 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator and N,N'-methylenebisacrylamide as a crosslinker. The IPN hydrogels were analyzed for the sorption behavior of water at 35degreesC and a relative humidity of 95% with a dynamic vapor sorption system, and water diffusion coefficients were calculated. Differential scanning calorimetry was used to quantitatively determine the amounts of freezing and nonfreezing water. The free-water contents in the IPN hydrogel samples PV51, PV31, and PV11 were 74.40, 64.03, and 60.48% in pure water, respectively. (C) 2003 Wiley Periodicals, Inc.