The osmotic pressure of aqueous solutions of poly(ethylene glycol) (PEG) and poly(N-isopropylacrylamide) (PNIPA) was measured over the temperature range from 5 to 35-degrees-C at concentrations from 0.5 to 2.5 wt.%. It was found that the mixing enthalpy DELTAh and mixing entropy DELTAs of water, which were determined from the temperature dependence of the osmotic pressure, were negative and that the magnitude of DELTAh was slightly larger than that of TDELTAs. This indicated that the dissolution of the polymers in water was dominated by enthalpy or intermolecular interaction. The existence of local structure in the solution was revealed by a negative entropy. Virial coefficients were determined from the concentration dependence of the osmotic pressure. The second virial coefficient of PEG aqueous solution increased linearly with a decrease in temperature, while that of PNIPA aqueous solution had a maximum around 15-degrees-C, and suggested the existence of two theta temperatures. The third virial coefficient of PNIPA aqueous solution increased at lower temperatures. The osmotic pressure data were correlated with the lattice model proposed by Prange et al. (Prange, M.M., Hooper, H.H. and Prausnitz, J.M., 1989. AIChE J., 35 : 803).