The processes in a liquid pre-wetted porous membrane are described for the situation when a gradually increasing pressure difference of an inert gas is imposed across the membrane (liquid expulsion permporometry). The description takes into account the compressible nature of the gas and the distribution of the pore sizes. By comparing the experimentally obtained dependence of the gas flowrate through the membrane versus the imposed pressure difference (wet curve) it is possible to evaluate the distribution of the fractional membrane porosity (pore-volume distribution) with the pore size. At the same time the ratio of porosity and tortuosity of transport pores (geometric membrane parameter, psi) is determined. This parameter can be used together with the mean integral pore radius [r] and the mean integral square of pore radii [r2] (determined by analysis of the gas flowrate dependence versus deltap for a dry membrane - dry curve) for the prediction of diffusion and/or viscous flow for any gas or gas mixture under any temperature and pressure condition. The simulated pore-size distributions are compared with the results of the traditional method; it appears that the traditional approach leads to systematic deviations in the obtained pore-size distributions.