A modified technique for the analysis of diffusion permoporometric data obtained with asymmetric membranes has been described. The modifications allow better estimates of pore sizes and pore size distributions 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 the cylindrical pore model. The extent of this overprediction is directly proportional to the asymmetry of the membrane and the thickness of its active layer.