A methodology was developed for characterizing a series of membranes with pore size less-than-or-equal-to 40 angstrom. These membranes were modified from commercial microporous gamma-Al2O3 membranes (40 angstrom) by pore size reduction. SEM/EDAX analysis showed a modified layer with a thickness of approximately 1.5 mum deposited within the existing porous structure. A flow-weighted pore size distribution analyzer was used to determine pore size versus flow contribution for the pore size ranging from 40 angstrom down to approximately 15 angstrom. Size exclusion with selected gaseous molecules was employed to determine pore size < 6 angstrom. Pore size not accounted for by the above two methods was assigned to 15-6 angstrom. Gas permeation of helium and nitrogen through the pore size ranging from > 40 to less-than-or-equal-to 5 angstrom at 25-degrees-C was investigated in detail. Selectivity decreased slightly along with the pore size reduction in the Knudsen regime (i.e., 40 angstrom down to approximately 20 angstrom), then increased dramatically in the molecular sieving regime. The unexpected decrease of the selectivity in the Knudsen regime was likely to have resulted from the contribution of surface diffusion of nitrogen.