Hydrogels with varying cross-linking densities and average molecular weights between two consecutive cross-links were prepared from the binary poly(N-vinyl-2-pyrrolidone)/water and ternary poly(Nw-vinyl-2-pyrrolidone)/water/K2S2O8 systems by irradiation with gamma rays at ambient temperature. Both hydrogel systems were employed for diffusion and swelling experiments in various urea solutions at room temperature. Diffusion of urea solutions into hydrogels has been found to be of the non-Fickian type, except for the samples of PVP/K2S2O8 hydrogels irradiated at higher doses in dilute urea solutions. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate constant, equilibrium water/urea content, and diffusion constant values were evaluated for PVP and PVP/K2S2O8 hydrogel systems at two different urea concentrations. With regard to swelling data, it was determined that the dilute urea solution tan penetrate much more effectively into the PVP/K2S2O8 hydrogel system. (C) 2000 John Wiley & Sons, Inc.