Partitioning of lysozyme, bovine serum albumin, and alpha-amylase in aqueous two-phase systems of PEG + K2HPO4 + water and PEG + Na2SO4 + water at 298 K was investigated. The effects of pH and salt concentration on protein partitioning were studied. The protein partitioning depends on the protein net charge that was determined experimentally by combining titration and isoelectric focusing. The experimental data are correlated using the VERS model, that is, an osmotic virial equation with relative surface fractions to express the concentrations. The protein is split into different patch groups, and the number of patches is calculated from the diameter of the hard sphere protein molecule. The comparisons between the correlation results and the experimental data reveal a good agreement.