The factors responsible for the dependence of membrane permeability on salt concentration, for low-pressure hydrous zirconium oxide-polyacrylic acid (Zr-PAA) dynamically-formed membranes (DFMs), have been studied. Data produced for Zr-polymer DFMs at different ionogenic site concentrations suggest that (a) the contribution of the hydrous zirconium sub-layer to investigated trends is minimal, (b) increasing salt concentration promotes dissociation of the polyacrylic acid for Zr-PAA DFMs at neutral pH, and (c) a reduction in the adsorbed PAA leads to an increased degree of molecular dissociation for a given pH. The expected increase in real rejection on increasing the membrane charge density was observed as being nullified by a concomitant decrease in permeate flux. Further processing of the data showed the changes in observed rejection to be attributable to changes in the degree of concentration polarisation.