Temperature-sensitive poly (N-tert-butylacrylamide-co-acryl-amide) [P(NTBA-co-AAm)] hydrogels were synthesized by free-radical copolymerization in a water-methanol mixture using three types of cross-linkers: 1,2-ethyleneglycol dimethacrylate, N,N-methylenebisacrylamide, and 1,3-butandiol dimethacrylate. These thermosensitive hydrogels were swollen to equilibrium in water at 20 degrees C and examined by gravimetric measurements. The influence of type and content of crosslinkers on the swelling ratio, the polymer-solvent interaction parameter (chi), the average molecular mass between crosslinks ((M) over bar (c)), and the effective crosslinking density (nu(E)) of the hydrogels were reported and discussed. The swelling process in water was found to be non-Fickian diffusion. The enthalpy (Delta H) and entropy (Delta S) changes appearing in the chi parameter for the hydrogels were determined by using the FloryRehner theory based on the phantom network model of swelling equilibrium. Negative values for Delta H and Delta S indicated that the hydrogels had a negative temperature-sensitive property in water; that is, swelling at a lower temperature and shrinking at a higher temperature. The temperature-reversibility and on-off switching properties of the P(NTBA-co-AAm) hydrogels may be considered as good candidates for designing novel drug-delivery systems.