Experimental data of permeate solute concentrations and fluxes, generated by nanofiltration (NF) tests of solutions of neutral solutes, are used as boundary conditions in computer fluid dynamics to predict the solute concentration at the feed side of the membrane surface, in a NF laboratory slit. This procedure allows for the determination of the experimental intrinsic rejection coefficients, f'(exp), as a function of the transmembrane pressure. The minimization of the squares of the deviations between the experimental and the steric pore flow model predictions of intrinsic rejection coefficients produces an average pore radius of 0.52 nm for the CDNF501 membrane. The results relative to the NF permeation of NaCl aqueous solutions exhibited a marked dependence on the solute concentrations. This is attributed to diffusive and convective restrictions to ion flux that can be explained by the electrostatic membrane-ion interactions developed in the NF membrane pores.