A statistical mechanical model for the component chemical potentials has been developed to predict the behavior of multiphase aqueous salt-polymer mixtures. The model includes the effect of polymer molecular mass on system phase behavior by means of scaling laws. The model parameters have been evaluated from binary and ternary isopiestic water activities and from solution volumetric data. Calculated phase diagrams for five different systems comprising three different polyethylene glycols and three different salts are shown. Model calculations show generally good agreement with experimental results.