A gas-liquid displacement method is investigated for characterizing pore-size distributions of microfiltration (MF) membranes. The complexities of using gas as displacing fluid are analyzed in terms of the gas transport regimes involved, showing that slip flow features gas transport in MF membranes, and Knudsen diffusion can be effectively suppressed by an appropriate selection of the pore-filling liquid. A universal mathematical model for evaluating pore-size distributions is developed based on the fundamental equation governing slip flow. An approximate model is also presented where only the Hagen-Poiseuille regime is considered. The pore-size distributions of two MF membranes are evaluated using the universal and the approximate models, respectively. It is found that the approximate model overestimates the probability of the larger pore (relative to the peak pore) and underestimates the probability of the smaller pore. It is recommended that the universal model be used if no priori analyses on gas transport regimes is carried out. (C) 2004 American Institute of Chemical Engineers.