In this study, water activities for the ternary systems of PEG(6000) + sodium citrate (Na(3)Cit) + H2O and PEG(6000) + potassium citrate (K(3)Cit) + H2O have been measured using an improved isopiestic method over the entire homogeneous mixing range at 298.15 K. From these measurements, values of the vapor pressure of solutions were also determined. The experimental data for the activity of water, collected in this work, were accurately correlated with local composition based models of the NRTL, modified NRTL, Wilson and modified Wilson. The results showed that these models can accurately correlate the water activity data generated in this work. (c) 2007 Elsevier B.V. All rights reserved.