Flexible random-coil structure of polymer molecules and protein-polymer and polymer-solvent interactions were accounted for in the evaluation of the interaction potential between two globular protein molecules which consisted of attraction because of depletion of polymer segments in the vicinity of protein and electrostatic repulsion because of the net charge of protein. The Gibbs free energies of protein in the solid and liquid phases, evaluated using a second-order perturbation theory around hard sphere as the reference, were equated to predict the phase diagram. Protein solubility was found to be lower and the concentration of protein in the solid phase was higher for higher polymer concentrations, larger protein sizes, higher molecular weight of polymer, weaker protein-polymer interactions, poorer solvent, higher ionic strengths, and pH values closer to the isoelectric point. Experimental measurements of solubility of bovine serum albumin, human serum albumin, and ovalbumin for different molecular weights of poly(ethylene glycol) agreed well with the model predictions.