Liquid-liquid displacement porometry (LLDP), is proposed to estimate the molecular weight cut-off value of Ultrafiltration (UF) membranes. Several commercial UF membranes are analysed by using LLDP and their pore size distributions have been used to estimate the molecular weight cut-off as should be obtained by dextran retention. Results compared reasonably with nominal cut-off values given by manufacturers. The method offers a fast and accurate way to assign cut-off values for UF membranes, without having to perform expensive and time consuming solute retention tests, which bring results very often difficult to compare due to the difficulties in the standardization of such methods. (C) 2010 Elsevier B.V. All rights reserved.