Phase diagrams of PPG425 + Na2SO4 + H2O and PPG425 + (NH4)(2)SO4 + H2O systems at 298.15 K were measured. The densities and viscosities of two-phase systems were also measured. The improved regular solution theory was used to correlate the equilibrium data. In this theory a mixing entropy term and the Fowler-Guggenheim long-range electrostatic term has been used. (C) 2003 Elsevier B.V. All rights reserved.