Several UF membranes with fairly different structure have been characterized by using a liquid-liquid displacement porometer, which has been developed under a close collaboration of our laboratories. Three polysulfone membranes from Millipore and three polycarbonate membranes from Nuclepore-Whatman were studied. The results of pore size distributions showed nice reproducibility and accuracy. The technique is also able to accurately evaluate membrane porosity for membranes with simple cylindrical pore-like structure. In addition the possibility to estimate MWCO, by LLDP results, makes the technique very useful for process selection. Finally SEM pictures of the Millipore membranes show that usual porosimetric liquids do not alter membrane structure.