In this study the effects of ionic strength and PH on ultrafiltration of protein solutions are examined. Experiments are done with BSA/NaCl solutions with PH varying from 3 to 8 and ionic strength from 5 to 150 mM. Fluxes depend on solution conditions and increase with decreasing ionic strength. They show a minimum at the iso-electric point. At pH values above the iso-electric point rejection of chloride is negative; this is due to electrostatic repulsion of the chloride ions by the negatively charged protein molecules. At pH 3, below the iso-electric point, the chloride rejection is positive. In this case the chloride ions are attracted by the protein., which is now positively charged. We describe our results by a model based upon the Maxwell-Stefan theory of mass transfer. This model takes electrical effects into account. Non-idealities are described using the Camahan-Starling equation of state for rigid spheres. Rejections and fluxes are well described by the model.