A quantitative analytical method for the study of the liquid-liquid equilibria (LLE) of K2HPO4-KH2PO4-PEG-protein aqueous two-phases systems has been established. 23 partition coefficients for six proteins at 25 degrees C were measured. The Baskir adsorption lattice model and Fitter long-range electrostatic term were extended to calculate the protein partition coefficients in aqueous two-phase systems containing electrolyte and polymer. Two adsorption energy parameters were regressed from six sets of experimental data of lysozyme-containing systems and can predict the other 17 partition coefficients of lysozyme and five other proteins in aqueous two-phase systems. The average relative deviation of the partition coefficient between experiment and calculation is less than 6%. The factors affecting protein partition coefficient were discussed.