It is shown that the introduction of imperfections into the model of nanofiltration helps to explain an apparent contradiction between the moderate reflection coefficients and rather large initial slopes of rejection versus transmembrane volume flow curves often observed in nanotiltration. Within the scope of conventional model with no account of imperfections, this mismatch between the reflection coefficient and solute permeability automatically brings about either strongly reduced diffusivities of solutes in pores or unphysically large effective active layer thicknesses. The model can also explain the relatively weak dependencies of salt rejections on the feed concentrations especially often observed for the salts with multiple-charge co-ions. (C) 2004 Elsevier B.V. All rights reserved.