The densities and viscosities of binary and ternary solutions of the poly(ethylene glycol) 4000 (PEG4000) + diammonium hydrogen phosphate + water system were determined at different temperatures (298.15, 303.15, 308.15, 313.15, and 318.15) K. The measured density and viscosity data of all the binary and ternary systems were fitted to available empirical correlations, for the corresponding temperatures. The density data show a linear variation with mass fraction of the polymer for all temperatures. The viscosity data of all the solutions were correlated as a function of their mass fraction, using a nonlinear equation, for the five different temperatures covered in the present work. Densities and viscosities of PEG4000 - diammonium hydrogen phosphate two-phase systems have been measured at (298.15, 303.15, 308.15, 313.15, and 318.15) K. The tie line lengths (TLL) of the aqueous two-phase systems have also been estimated, and the effect of the physical properties on the TLL is also reported. © 2009 American Chemical Society.