In the present work the Flory-Huggins model with two electrostatic terms (the Debye-Huckel equation and Pitzer-Debye-Huckel equation) and the osmotic virial model have been used to obtain the activities of polymer, salt and water for the liquid-liquid equilibrium of the aqueous polyvinylpyrrolidone + potassium dihydrogen phosphate/dipotassium hydrogen phosphate buffer at various temperatures (298.15, 308.15 and 318.15 K) and pH 5 and 7. The effects of the temperature and pH on the activities of polymer, salt and water have been investigated. To investigate the predictive performance of these models, the energetic interaction parameters were estimated by minimizing the proper objective function using the singular value decomposition method. The very low deviations obtained suggested that the models studied provide a very good predictive description of the system investigated. Crown Copyright (c) 2007 Published by Elsevier B.V. All rights reserved.