A model is developed which describes mass transfer through the combination of convective-diffusive flux in the pores and a diffusive flux in the membrane material. This model takes into account membrane (membrane permeability, pore size) and solution (charge and size of the molecule) characteristics via five parameters which can be estimated from experimental data. This model is first validated by comparison with experimental results and then used as a tool to evaluate the influence of the membrane solution characteristics on the performance of nanofiltration process.