The potential of using ultrafiltration for separation of salt solutions has been explored. Solutions of phosphates were filtered through commercially available ZrO2 ultrafiltration membranes, with a cut-off value of 15 kD. In the experiments, effects of cross flow, permeate flux, pH and ionic strength of the solution on rejection were the main items of interest. The process is modelled using the Maxwell-Stefan equations for mass transfer, accounting for the three different driving forces that govern the process (gradients in electrical potential, pressure and concentrations). The rejections observed for the phosphate ions were surprisingly high (up to 80%) considering the cut-off Value of the membrane used. They were also strongly influenced by the ionic strength of the solution, indicating that electrical effects are important. The rejection curves are well described by the Maxwell-Stefan model, in which the charge of the membrane was assumed to be dependent upon solute concentration according to a Freundlich isotherm. The model is also able to describe the effect of concentration polarisation in the liquid boundary layer in front of the membrane.