The effect of the concentration boundary layer on the pressure-driven membrane separation like nanofiltration or ultrafiltration is well-known and extensively discussed in the literature. In most of these studies, the effect of the boundary layer and that of the membrane layer on the separation efficiency are discussed separately. This paper presents a general model to describe the convective and diffusive mass transport taking into account the simultaneous effect of both the concentration boundary layer and the membrane layer. It has been demonstrated how the Peclet number of the membrane, the ratio of diffusive mass transfer coefficient of the membrane to that of the polarization layer and the product of the steric partition coefficient and the convection hindrance factor affect the permeate concentration or the rejection coefficient. Comparing the measured and calculated data, it has been proven that the effect of the polarization layer can significantly affect the separation efficiency during practical nanofiltration of uncharged compounds. The model developed gives a general treatment method to evaluate pressure-driven membrane separation processes. (C) 2010 Elsevier B.V. All rights reserved.