Abstract In this work, the pore size distributions referred to volume, surface and number of pores, along with the internal surface area and the surface roughness parameters are determined for a capillary microporous symmetric membrane with a nominal diameter of 150 Å. This has been realized by using two specially interesting techniques: nitrogen adsorption-desorption at 77 K and atomic force microscopy (AFM). Adsorption isotherms, combined with the BET theory for multilayer adsorption allowed us to obtain the internal surface area of the membrane, while the volume, surface and pore number distributions were calculated from the Kelvin equation both in the adsorption and desorption processes. In this way the existing asymmetry between both adsorption and desorption processes was analyzed and it has been related to the dead-end pores (pores closed to flux). The results of image analysis on AFM micrographs allowed the determination of the surface roughness parameters of the membrane, along with the pore sizes on the membrane surface. Finally, the results have been thoroughly analyzed and compared with other characterization results previously obtained for these membranes from different characterization techniques by other authors.