Water activities of the polyvinylpyrrolidone (PVP) + (NH4)(2)SO4 + H2O system have been determined experimentally using the improved isopiestic method at 298.15, 308.15 and 318.15 K. The effect of temperature on the constant water activity lines has been studied. It was found that the slope of all constant water activity lines increased with increasing temperature. Furthermore, the extended Wilson theory has been used for the correlation of the experimental water activity data. The agreement between the correlation and the experimental data is good. (c) 2005 Elsevier B.V. All rights reserved.