Recent studies have demonstrated the feasibility of using membrane ultrafiltration for the purification of pegylated proteins; however, the separations have all been performed at relatively low protein concentrations where intermolecular interactions are unimportant. The objective of this study was to examine the behavior at higher PEG concentrations and to develop an appropriate theoretical framework to describe the effects of intermolecular interactions. Ultrafiltration experiments were performed using pegylated -lactalbumin as a model protein with both neutral and charged composite regenerated cellulose membranes. The transmission of the pegylated -lactalbumin, PEG, and -lactalbumin all increase with increasing PEG concentration due to the increase in the solute partition coefficient arising from unfavorable intermolecular interactions in the bulk solution. The experimental results were in good agreement with a simple model that accounts for the change in Gibbs free energy associated with these intermolecular interactions, including the effects of concentration polarization on the local solute concentrations upstream of the membrane. These intermolecular interactions are shown to cause a greater than expected loss of pegylated product in a batch ultrafiltration system, and they alter the yield and purification factor that can be achieved during a diafiltration process to remove unreacted PEG. (c) 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:655-663, 2013