The research hypotheses for this study were based upon the assumptions that negatively charged polymeric membranes have a dynamic permeability controlled by intramembrane charge repulsion, and that electrostatic forces at the membrane-solution interface contribute to rejection of co-ion solute species. The term membrane permeability is used here to include both permeate flux and solute permeation behavior. The objectives of this research were to study: the effect of feed-water ionic character on membrane permeability in the absence of organic material (experimental variables of pH and ionic strength); the effect of NOM solute concentration on membrane permeability and solute accumulation on the membrane surface for feed-waters of different ionic character; and the difference in solute permeation for charged (anionic) and uncharged (non-ionic) organic solute species in feed-waters of different ionic character.