Water activities of polypropylene glycol (PPG425) + K2CO3 + H2O and PPG425 +Na2CO3 + H2O systems have been measured using isopiestic method at 298.15 K. The water activities were correlated successfully with segment-based local composition Wilson model and extended form of Chen-NRTL models. The Pitzer-Debye-Hackel (PDH) equation and the Flory-Huggins expression were, respectively, used for the long-range interactions and the combinatorial contribution in the models. The parameters of the models for these systems were calculated. Good agreement was obtained with the experimental water activity data with both models. (c) 2005 Elsevier B.V. All rights reserved.