Transport properties of membranes are closely related to morphological properties like surface porosity and variation of their inner pore structure. Scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) are powerful tools to characterise the microscopical pore structure of membranes in a qualitative manner. In order to provide more quantitative data of surface and cross-sectional pores computer image analysis can be used. Parameters like 'porous area fraction' and 'mean free path length' have been selected to describe the pore distribution within porosity profiles in order to consider the effect that the pores within the cross-section are connected to each other. In this paper the complete method is described starting with the sample preparation, investigation with SEM and TEM and subsequent computer image analysis. Results obtained from different polyethersulfone (PES) based microfiltration membranes are presented. The porosity profiles describe the local pore variation within the membrane quantitatively, generating additional information about the functionality of the filtration process. It has to be seen as supplementation to usual 'bulk' test methods, e.g. bubble point. (C) 2008 Elsevier B.V. All rights reserved.