Temperature sensitive poly{N-[3-(dimethylaminopropyl)]wmethaerylamide-co-acrylamidel [P(DMAPMA-co-A-Am)] hydrogels were prepared by the free-radical crosslinking copolymerization of corresponding monomers in water with N,N-methylenebisacrylamide as the crosslinker, ammonium persulfate as the initiator, and N,N,N',N'-tetramethylethylenediamine as the activator. The swelling equilibrium of the P(DMAPMA-co-AAm) hydrogels was investigated as a function of temperature in aqueous solutions of the anionic surfactant sodium dodecyl sulfate and the cationic surfactant dodecyltrimethylammonium bromide. In pure water, regardless of the amount of N,N-methylenebisacrylamide, the P(DMAPMA-co-AAm) hydrogels showed a discontinuous phase transition between 30 and 36 degrees C. However, the transition temperature changed from discontinuous to continuous with the addition of surfactants; this was ascribed to the conversion of nonionic P(DMAPMA-co-AAm) hydrogels into polyelectrolyte hydrogels due to the binding of surfactants through hydrophobic interactions. Additionally, the concentrations of free sodium dodecyl sulfate and dodecyltrimethylammonium bromide ions were measured at different temperatures by conductometry, and it was found that the electric conductivity of the P(DMAPMA-co-AAm)-surfactant systems depended strongly on the swelling ratio; most notably, it changed drastically near the phase-transition temperature of the P(DMAPMA-co-AAm) hydrogel. (c) 2006 Wiley Periodicals, Inc.