The porous support layer of a high-flux composite membrane can have an influence on gas or vapor separation performance. In order to quantify that influence, the theory of gaseous flow through porous media was reviewed and applied to describe transport through the porous layer. The flux equations may be used to characterize the morphology of the support structure. An experimental technique was developed to determine morphological parameters based on single gas permeation experiments. Flat sheet and hollow fiber support membranes were tested and compared. Due to the axial pressure drop inside the hollow fibers, an appropriate average lumen pressure had to be determined. As a by-product, the inside diameter of the porous hollow fibers may be estimated from the permeation experiments. The calculated morphological parameters varied slightly with six different gases used, but these differences could be reconciled when the asymmetry of the porous structure was taken into account. The support membranes could be modeled accurately by a two-layered structure and a resistance in series approach. By combining the description of gaseous flow in the porous support presented in this paper with standard transport equations for the dense coating, it should be possible to describe binary gas transport through a composite membrane and, ultimately, to determine the influence of the support layer.