The objective of this study was to investigate distinctive reverse osmosis (RO)/nanofiltration (NF) membrane characteristics that could be used to describe the porosity of the membrane's active layer without resorting to theoretical models. From the membrane's flux dependency on pressure, two characteristic parameters were determined: (1) the susceptibility of the active layer structure under pressure and (2) the membrane's pure water permeability. The significance of the characteristic parameters was confirmed by relating them to the pore size distributions (PSD) and to the effective number of pores, N in the active membrane layer. The examined RO/NF membranes were classified according to the values of the characteristic parameters, and the classification was illustrated by determining the membranes' rejection properties for various, mostly organic solutes. (c) 2006 Elsevier B.V. All rights reserved.