A modified technique for the characterization of membranes by liquid displacement permoporometry has been described. The modifications allow an estimate of the shape of a pore in an asymmetric membrane to be made. The erroneous nature of the conclusions that would be drawn if permoporometric data, obtained with a membrane with asymmetric pores, is treated under the assumption of right cylindricity has been demonstrated by means of such an analysis done on a hypothetical asymmetric membrane. While there is little error in the estimation of pore radii, the pore numbers are grossly overpredicted by a right cylindrical pore model for the membrane. The extent of this overprediction increases with the asymmetry of the membrane.