The interactions between globular proteins in the presence of poly (ethylene glycol) (PEG) are probed through the measurement of the protein solution second virial coefficient (B-2) The solution properties of PEG are characterized for four molecular weights (400, 1000, 6000, and 12 000), providing an opportunity for quantitative comparison of measurements and theoretical predictions of B-2 PEG displays a buffer and molecular weight-dependent lower critical solution temperature. As the polymer solution approaches phase separation, the consequences of depletion attractions increase significantly. For lysozyme and bovine serum albumin in sulfate buffers with PEG, B-2 is not well described by standard depletion models. This failure is accentuated in acetate buffers where B-2 is a nonmonotonic function of polymer concentration. The attractive minima in B-2 are closely associated with the proximity of the heating-induced phase separation of aqueous PEG solutions. The experimental data for both proteins in the presence of PEG are well captured by the thermal polymer reference interaction site model for depletion interactions where the polymer density fluctuation correlation length is treated as a function of temperature, polymer concentration, and molecular weight.