The water activities of aqueous polypropylene oxide 400 solutions in the polymer mass fraction range 0.1-0.9 were measured by the isopiestic method at 293.15, 298.15, 303.15, 308.15, 313.15 and 318.15 K. The data obtained were used to calculate vapor pressure and osmotic coefficient of solutions and activity coefficient of solute and solvent at 293.15, 298.15, 303.15, 308.15, 313.15 and 318.15 K as a function of concentration. A comparison was made with the results for aqueous solutions of polyethylene oxide that is structurally closely related to polypropylene oxide. The effect of temperature on the vapor-liquid equilibrium of polypropylene oxide + H2O systems has been studied and the results have been discussed on the basis of the effect of temperature on the hydrophobicity of the polymer. Furthermore, the segment-based local composition Wilson model has been used for the correlation of the experimental water activity data and the obtained model parameters have been used for predicting the excess molar enthalpy of aqueous polypropylene oxide solutions. The agreement between the prediction and the experimental data is good. (c) 2006 Elsevier B.V. All rights reserved.