The phase transition of gels containing N-isopropylacrylamide (NIPA) was investigated by differential scanning calorimetry (DSC) and swelling measurements. The enthalpy of dissociation of the hydrophobic interaction per molar unit of NIPA chains (Delta H-N), the transition temperature (T-c), and the number of water molecules associated with an NIPA monomer, (n) were evaluated as a function of polymer concentration. Significant differences in Delta H-N and T-c were found between two systems : poly(NIPA-ran-acrylic acid) (NIPA/AAc; a weakly charged gel) and poly(NIPA-ran-dimethylacrylamide) (NIPA/DMAA; a neutral gel). Delta H-N decreases with increasing comonomer concentration. However, a larger decrease in Delta H-N was observed for NIPA/AAc than for NIPA/DMAA, which is accounted for by the strong hydrophilic effect of the charged AAc comonomers. No noticeable copolymer concentration dependence in n was observed in both systems. It is suggested that there are two types of water molecules, i.e., one associated with the phase transition, (n - n(0)), and the other the lower limit for the hydrophobic hydration, n(0). The roles of these water molecules are discussed in relation to the volume phase transition.