Phase diagrams of aqueous two-phase systems composed of polyethylene glycol with average molecular mass 6000 and various salt solutions were studied experimentally. The salts used were magnesium sulfate, sodium sulfate and ammonium sulfate. The salting-out power of salts in these systems were determined and this power can be arranged with respect to the cations as Mg2+ > Na+ > NH4+. A statistical mechanical model, developed by Cabezas et al. [Fluid Phase Equilib. 53 (1989) 453] was used for the prediction of equilibrium data. This model includes the effect of polymer molecular mass on system phase behavior by means of scaling laws. Model calculations show good agreement with experimental results.