In this work, the pore size distributions referred to volume, surface and number of bulk pores, along with the internal surface area and the size parameters of surface pores are determined for two polyethersulphonic microporous composite membranes of nominal MWCO of 4000 and 30000 Da. The microporous support pore size distribution has also been obtained. These goals have been achieved using several characterization techniques : atomic force microscopy (AFM), N-2 adsorption-desorption at 77 K and a liquid displacement technique. A computerized analysis of the AFM micrographs allowed determination of the pore size distribution of surface pores at different magnifications. Also, surface roughness can be obtained. Adsorption isotherms, combined with the BET theory for multilayer adsorption, allowed determination of the internal surface area of the membrane, while the volume, surface and pore number distributions were calculated from the Kelvin equation, both in the desorption process to obtain the so-called mesopore distribution. Further analysis, by extending the pore size analysis to pores where the Kelvin equation is not valid, allowed determination of the micropore distribution. Finally, the membrane support has been detached and analyzed by a modified bubble point or liquid displacement technique. Analysis and comparison of all results show that several pore populations (including micro- and mesopores) are present in the membrane with considerable differences between surface pores and bulk pores, the latter being those which should determine permeation. Two further mese- and macropore populations could be assigned to the membrane-support transition or the support itself.