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An asymmetric nanofiltration membrane (Hoechst, PES5) has been characterised by three different techniques: modelling of the rejection of simple salts, modelling of the rejection of uncharged solutes and atomic force microscopy. Interpretation of experimental data for the rejection of three salts having common co-ion (LiCl, NaCl, KCl) with model calculations allows a characterisation of the membrane in terms of three parameters: an effective pore radius (r_p), the ratio of effective thickness over porosity (Δx A _k) and an effective charge density (X). Interpretation of experimental data with model calculations for uncharged solutes (Vitamin B₁₂, raffinose, sucrose, glucose, glycerin) allows a characterisation in terms of r_p and Δx A _k. Atomic force microscopy (AFM) allows direct determination of surface pore radius r_p^s and surface porosity A_k^s. The AFM images provide direct confirmation of the presence of discrete surface …