The effect of added salt (NaCl or KCl) on the polyethylene glycol 8000 (PEG) sodium sulfate (Na(2)SO(4)) aqueous two-phase system (ATPS), containing 0.01 mol.L(-1) of sodium phosphate buffer (NaPB), pH 7.4, has been investigated at 296.15 K. Phase diagrams determined by the cloud point method, including tie-lines assigned from mass phase ratios according to the lever arm rule, are presented for the different PEG 8000-Na(2)SO(4) ATPSs, all containing 0.01 mol.L(-1) NaPB, pH 7.4, and increasing the concentration of a neutral salt, such as NaCl or KCl, up to 1.0 mol.L(-1). Experimental binodal results were satisfactorily correlated with an empirical mathematical model, the Merchuk equation. The results indicate that the addition of both salts causes a depression of the binodal relatively to that for the salt-free system. The salting-out ability of the cations follows the Hofmeister series (Na(+) > K(+)) and can be related to the ions Gibbs free energy of hydration (Delta G(hyd)).