Isopropanol displacement under nitrogen pressure was used for the determination of pore size distribution in microfiltration polypropylene hollow fibres. Applying various assumptions about gas transport process two completely different characteristics of pore sizes were obtained. To verify these results an analysis of SEM images of the investigated membrane was conducted concerning its porous structure (pore diameters, surface occupied by pores). According to the SEM analysis the mean coverage of membrane surface by pore entrances should be about 20% of total area. For the distribution which accounted for pore evacuation according to Young-Laplace equation with contact angle theta = 67 degrees surprisingly dense coverage amounting to over 70% of total surface (by calculated total pore number over 10(13) per m(2)) was predicted. Results for the distribution which accounted for gas bubble formation, at the membrane surface (equivalent to theta = 0 degrees) fit into the expected range of pore numbers and membrane coverages (about 10(11) per m(2) and about 10%, respectively). It is concluded that the mechanism of bubble formation, determined by an actual pressure, liquid surface tension and pore size, is the crucial process while the value of contact angle theta does not play any role in the determination of pore size distribution.