Copolymer hydrogels were prepared through the gamma irradiation of aqueous solutions composed of different ratios of acrylamide (AAm) and vinyl pyrrolidone (VP) monomers. The chemical structure, thermal stability, and structural morphology of the hydrogels were characterized with Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy, respectively. The IR spectroscopy analysis showed the formation of copolymerization and the presence of hydrogen bonding. The TGA study showed that the AAm/VP-based hydrogels possessed higher thermal stability than polyacrylamide (PAAm). However, the thermal stability of the AAm/VP hydrogels increased with an increasing ratio of the VP component. The study of the swelling kinetics in water showed that all the hydrogels reached the equilibrium state after 24 h. However, the AAm/VP-based hydrogels showed swelling in water that was lower than that of the hydrogel based on pure AAm. Meanwhile, the degree of swelling of the AAm/VP-based hydrogels decreased with an increasing ratio of VP in the feeding solutions. The results showed that the PAAm and AAm/VP-based hydrogels prepared at 50 kGy were affected by a change in the temperature around 25 degrees C, whereas the hydrogels prepared at 25 kGy did not show this characteristic. However, the hydrogels prepared at different doses displayed reversible pH character. (C) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 106: 4202-4208, 2007