The aim of this work is to study the transport mechanism of ions through nanofiltration membranes. A model based on extended Nernst-Planck and film theory equations is reported. This model can be characterized by three transport parameters: the water permeability L-p, the salt transmittance Phi and the effective salt transfer coefficient K-eff. The knowledge of the feed and permeate concentration and of the permeate volumetric flux enable us to calculate these transport parameters. The model is used to estimate cadmium salts rejection by a NANOMAX 50 membrane. Experimental and calculated results are shown to be in good agreement. The model is then successfully extended to experimental data reported in the literature.