The development of microporous inorganic membranes for gas separation depends crucially on the accurate characterization of gas transport through such membranes. Particularly in the initial development stages, in which low membrane selectivities are common, measurement of pure gas permeance as a function of mean pressure can provide a useful characterization tool. For example, this method can be used to obtain information regarding pore size reduction in microporous membranes. Unfortunately, due to demanding limits on experimental error and misinterpretation of data, this method has been often applied incorrectly. In this paper, a theoretical basis for describing gas transport through both monolithic and multilayer porous systems is described. Implications of this formalism for practical data acquisition and analysis are discussed. Experimental equipment and methods are described for collecting data satisfying these strict criteria for data precision. The reproducibility of the experimental methods is illustrated by data obtained for a microporous membrane layer deposited on a porous support. Application of the present formalism to the prediction of transport characteristics for different gases, as well as to the detailed modeling of the pore structure of multilayer systems, is also illustrated by permeation data for several supported thin film microporous membranes.