The behaviours of two different nanofiltration membranes (NF45 and NF70) were studied. Experiments were carried out under various transmembrane pressures, concentrations and solutes (NaCl, Na2SO4 and PEG200) in the absence of the effect of concentration polarization. The experimental data were processed using the dynamic permeability model and the generalized Kimura-Sourirajan model. The experiments run with charged or uncharged membranes and solutes permit us to give a clear understanding of the relative contributions of steric and surface interactions on the separation characteristic. Furthermore, the experimental results show that there exists another type of surface force interaction other than the electric repulsion which can dominate the transport of solute. The results show that the strongly pronounced dependency of the separation factor, the dynamic permeability as well as the solute transport parameter on the electrolyte concentration, is only the characteristic of a charged membrane.