Gas permeation through a typical state-of-the-art membrane can be described by defining three morphological features: namely skin thickness, skin integrity, and substructure resistance. Traditional gas permeation measurements tend to characterize skin thickness and skin integrity, but not substructure resistance. This presents a serious obstacle to the optimization of advanced hollow fiber membranes, since as skin thicknesses are reduced, substructure resistance becomes an increasingly significant contribution to the overall permeation rate. This paper illustrates how substructure resistance can affect permeation properties and demonstrates a new technique for characterizing this frequently important morphological feature. The technique involves applying a constant transmembrane pressure while varying the average gas pressure within the membrane. Thus, the mean free path of gas molecules permeating through the substructure can be altered while maintaining a constant driving force for permeation. Such experiments characterize the magnitude of the substructure resistance, as well as provide insight into the governing transport mechanism. These constant driving force/variable pressure permeation measurements can estimate the average pressure or mean free path at the transition where substructure resistance becomes negligible. This can then be used to compare the morphological features of different membranes. This technique is demonstrated on well-defined coated ceramic membranes, asymmetric polymeric flat sheet membranes, and asymmetric polymeric hollow fiber membranes.