An extended bubble point method has been used to examine the porous morphology of several track-etched microporous polycarbonate membranes with nominal pore sits ranging from 0.1 to 5.0 mu m. The technique has been carefully analyzed and corrected to take into account the diverse non-ideal factors in flow along-with the prevalence of Knudsen flow over the Hagen-Poiseuille one in the smaller pores. The pore size distributions were obtained in terms of flow or pore numbers. Bulk porosity and pore densities were in good agreement with nominal and SEM values for each filter. Performance wise, measured water permeabilities agreed well with those obtained from pore density distributions, assuming Knudsen air flow through the narrowest pores.