In this study, poly(N-vinylpyrrolidone-co-methacrylic acid) (P(VP/MAA)) hydrogels prepared with gamma-radiation were used for swelling, diffusion, and uptake of some cationic dyes which are Janus Green B (JGB) and Magenta (M). P(VP/MAA) hydrogels irradiated at 3.4 kGy were employed for swelling and diffusion studies in water and cationic dye solutions. The influence of dose, pH, relative amounts of monomers in VP/MAA monomer mixtures on swelling properties were investigated. P(VP/MAA) hydrogels were swollen in distilled water at pH 4.0, 7.0, and 9.0. P(VP/MAA) hydrogel containing 5 mole% methacrylic acid showed the maximum % swelling at pH 9.0 in water. Adsorption isotherms were plotted for JGB and M in P(VP/MAA) hydrogel systems. Consequently, it may be concluded that crosslinked P(VP/MAA) hydrogels may be successfully used in the purification of waste water containing certain textile dyes.