A series of an ionic hydrogels composed of N,N-diethylaminoethyl methacrylamide (DEAEMA), N-vinyl-2-pyrrolidone (VP), and itaconic acid were synthesized by free-radical cross-linking copolymerization in water-ethanol mixture by using N,N-methylenebis(acrylamide) as the cross-linker, ammonium persulfate as the initiator, and N,N,N ',N '-tetramethylenediamine as the activator. The swelling behaviors of these hydrogels were analyzed in buffer solutions at various pH. It was observed that the swelling behavior of cross-linked ionic poly(N,N-diethylaminoethyl methacrylamide-co-N-vinyl-2-pyrrolidone) [P(DEAEMA/VP)] hydrogels at different pH agreed with the modified Flory-Rehner equation based on the affine network model and the ideal Donnan theory. The swelling process in buffer solutions at various pH was found to be Fickian-type diffusion. The pH-reversibility and on-off switching properties of the P(DEAEMA/VP) hydrogels may be considered as good candidate to design novel drug-delivery system. (c) 2005 Wiley Periodicals, Inc.