Three independent methods are used to get the pore size distribution of nanofiltration membranes. Two membranes from PCI - AFC-40 and AFC-80 -have been studied. The surface pore size distribution has been studied by AFM images of the membrane surface. A steric pore flow model with friction has been used for different neutral solutes to obtain the effective pore size seen by several organic uncharged solutes. This model is improved by taking into account the modifications of viscosity for confined geometries in narrow pores and by using the Hagen-Poiseuille model to isolate the pore radius as the only parameter of the model. The retentive fractions of pores for the solutes used, which sizes are over the effective pore size seen by them, have been considered to obtain the pore size distribution. Finally a liquid-liquid displacement technique has been used to directly get the open pore size distribution. A fairly good agreement has been obtained by all the three methods used. (C) 2007 Elsevier B.V. All rights reserved.