Membrane technology applied to whey protein separation is an interesting development that has seen growth in recent years. In particular, modification of existing membranes to impart charge properties on the membrane surface or in the pores has been shown to improve membrane selectivity, product purity, and throughput of protein solutions. This research focuses on exploring the effects of membrane charge and solution pH on filtration of the whey proteins alpha-lactalbumin (14 kDa) and beta-lactoglobulin (18 kDa). In general, protein species having the same charge as that of the membrane are retained due to electrostatic repulsion, while those which possess opposite charges to that of the membrane pass through as a result of electrostatic attraction. A pressure gradient applied across the membrane provides the driving force for separation. The membranes are chemically modified by polymerization of styrene in the pores of an ultrafiltration polyethersulfone membrane. The resulting polystyrene grafts are activated with sulfuric acid to produce numerous negatively charged sulfonic acid groups. The functionalized membrane gave a selectivity of 5 times compared to the raw membrane at pH 7.2. The enhanced selectivity of the tailor-made membrane was due to the adjustment in molecular sieving combined with electrostatic repulsion between negatively charged beta-lactoglobulin and the negatively charged membrane.