Supported carbon molecular sieve membranes (CMSMs) were prepared by the pyrolysis of a polyetherimide (PEI) polymeric precursor The membranes were characterized by scanning electron microscopy, energy dispersive spectroscopy, and micropore analysis, using gas adsorption techniques to relate their microstructure characteristics to their transport and separation characteristics. The analysis shows that preparation conditions determine whether the carbon molecular sieve layer forms within or outside the gamma -alumina layer of the substrate. Micropore CO2 and N-2 adsorption analysis of both supported and unsupported CMSMS showed a sharp peak at about (3.6-3.8) x 10(-10) m using the Horvath-Kawazoe method For the supported CMSMs it was observed that consecutive coating/carbonization steps reduced the pore volume in the micro- and mesoporous regions, without greatly affecting the volume of pores in the range of (3.5-6) x 10(-10) m. The reduction of the pore volume in the micro- and mesoporous regions is accompanied by an increase in the separation factor and a decrease in the permeance after each coating/carbonization cycle. Micropore analysis of a membrane, whose performance had degraded, indicated that its volume of pores between (3.6-6) x 10(-10) m had drastically decreased from the corresponding value of the as-prepared membranes.