The improved isopiestic method have been used to obtain activities of water for poly(vinylpyrrolidone) (PVP) + sodium citrate (Na(3)Cit) + H2O and Na(3)Cit + H2O systems at 298.15, 308.15 and 318.15 K. From these measurements, values of the vapor pressure of solutions were determined. The effect of temperature on the vapor-liquid equilibrium of PVP + Na(3)Cit + H2O systems has been studied. It was found that, the slope of constant water activity lines increased with increasing the temperature. Also it was found that the vapor pressure depression for an aqueous PVP + Na(3)cit system is more than the sum of those for the corresponding binary solutions. 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 NRTL and Wilson models have been used for the correlation of the experimental water activity data. The agreement between the correlation and the experimental data is good. (c) 2006 Elsevier B.V. All rights reserved.