Liquid-liquid equilibrium data and phase diagrams of aqueous two-phase systems (ATPS) composed by water + potassium chloride + poly(ethylene glycol) (PEG) + sodium polyacrylate (NAPA) of different molecular masses, (8000 or 15 000) g.mol(-1), were determined at specific pH (7.0 and 9.0). The effects of pH and NAPA molecular mass on the equilibrium data and tie-lines were studied. It was found that an increase in molecular mass caused a decrease in the two-phase region for the ATPS at pH 7.0, while at pH 9.0 no effect was observed. A change to the pH values of ATPS did not influence the equilibrium data of the systems studied. The universal quasichemical (UNIQUAC) activity coefficient model with the inclusion of a Pitzer-Debye-Huckel term was used to thermodynamically describe the solutions. The parameters obtained accurately reflected the experimental observations. The viscosity and density properties of the ATPS phases were obtained, and data were submitted to a linear regression analysis. A polynomial model presented good agreement with the experimental data.