A predictive model for the performance of a nanofiltration membrane in separating the components of a dye/salt solution was developed It is based on the extended Nernst-Planck equation with incorporation of concentration polarization for mixtures of charged ions. Excellent agreement between predicted and experimental rejections of anions and cations was obtained for batch nanofiltration at different ratios of dye/salt assuming that the membrane charge density (X-d) depends on the total concentration of negative charges in the solutions. Prediction for diafiltration experiments was also excellent by allowing for variation in X-d as the salt concentration in the solution changed as a function of time. The model was used to investigate optimization of the processing conditions. The overall diafiltration process operated best in a two-phase mode, in which the dye solution was preconcentrated to the final required dye concentration before diafiltration.