The aim of this study is to investigate the transport mechanism of NaCl through nanofiltration membranes in mixed solutions. A model based on the Spiegler-Kedem and Perry-Linder models has been developed. Concentration polarization effects of an organic ion and NaCl were not included in the previous studies. The effects of concentration polarization on salt removal were added to the model. Reactive Black 5 (RB5) was used as an organic ion in the rejection of NaCl by DS5 membrane in all experimental runs. Different concentrations of RB5 and NaCl have been studied and a coefficient of the gel polarization effect of dye on salt removal (alpha) was defined depending on molar concentration ratios of dye and NaCl. Experimental and calculated results were shown to be in good agreement.