Hydrogels were prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with sodium acrylate (SA) and sodium methacrylate (SMA) as comonomers. For each copolymerization three different compositions were used and for all systems three concentrations of a tetrafunctional crosslinker were employed. Swelling ratios, r = (mass hydrogel/mass dry polymer), were measured on heating from 20 degrees C and then on cooling back to 20 degrees C, and the swelling/deswelling was shown to be thermoreversible. Lower critical swelling temperatures (T-c) were exhibited in copolymers with low concentration of sodium salt. The type of comonomer did not affect the swelling-deswelling behaviour in hydrogels with low concentration of comonomer, but there were strong differences at higher concentrations. These variations were attributed to small differences in the degree of hydrolysis of the SA and SMA units within the hydrogel.