In order to investigate the influence of ionic interactions on protein ultrafiltration, sulfonated polysulfone and polyethersulfone membranes were coated with polyvinylimidazole (PVI) and quaternized by a cross-linking agent bearing Bisphenol A. The apparent PVI thickness. calculated from ATR-FTIR, was in the range from 10 to 40 nm for the polyethersulfone and sulfonated polysulfone membranes. Significant difference in protein retention (positive lysozyme and negative bovine serum albumin) was observed at low ionic strength for oppositely charged protein and (modified) membrane. At high ionic strength, protein retention and membrane flux with modified and unmodified membranes were similar due to protein adsorption on the membrane and interactions of adsorbed protein to free protein. Hydrophobic interactions between Bisphenol A and protein were demonstrated with similarly modified HPLC silica. Hence, the hydrophobic interactions could partly explain the unexpected high protein retention observed both with the unmodified and modified membrane at high ionic strength.