The rejections of mixtures of polyelectrolyte (sodium polyacrylate) and a monovalent salt (NaCl) were measured as a function of flux for two nanofiltration (NF) membranes. These rejections were corrected for concentration polarization of both salt and polyelectrolyte. The feed concentration ratio of polyelectrolyte to salt ranged from 2 to 20. The extent of reduction in monovalent salt rejection in the presence of polyelectrolyte could be predicted based on the Donnan effect after evaluating the unbound fraction of polyelectrolyte counterion, phi (P), in independent dialysis experiments. In the dialysis experiments it was found that beta was 0.32. From this the value of the Donnan partition coefficient, beta was evaluated allowing for the effect of concentration polarization of both polyelectrolyte and salt. The concentration polarization of polyelectrolyte resulted in beta changing as a function of flux.