Different types of flat and hollow fibre membranes have been imaged by means of both common scanning force microscopy and, a new application especially for porous materials, pneumatic scanning force microscopy. This technique enables us to obtain more specific information about the actual volume, the diameter and the distribution of pores through which the gas is streaming. A comparison of the silicon-nitride tip and the NanoProbe shows that the latter is the only one suited to obtaining clear images.