Membrane charge can have a large effect on the performance of many ultrafiltration processes. Previous studies of membrane charge have generally been performed using solutions of simple monovalent salts; however, the effective surface charge can be strongly influenced by the binding or adsorption to the membrane surface of specific buffer ions present in actual process solutions. Experimental data were obtained for the zeta potential of both standard and positively-charged polyethersulfone ultrafiltration membranes in the presence of a variety of monovalent and multivalent buffer ions. The zeta potential of the positively-charged membrane was very strongly dependent upon the buffer, with the adsorption of di- and tri-valent anions causing a large reduction in the zeta potential. Changes in buffer composition also caused long transients in the membrane zeta potential under some conditions due to the slow kinetics for the displacement of strongly bound ions from the membrane surface.