The temperature-sensitive poly(N-isopropylacrylamide) hydrogels, prepared by gamma and electron-beam (EB) irradiation, were studied using positron annihilation lifetime spectroscopy (PALS). The effect of water content in the hydrogel on the ortho-positronium (o-Ps) lifetime and intensity was investigated. The observed positronium lifetime suggests microstructural differences between gamma- and EB-synthesized hydrogels. The distribution in positronium lifetime indicates nonhomogeneity in the distribution of free-volume holes in EB-synthesized hydrogels.