This paper considers physical aging in carbon molecular sieve (CMS) membranes. Moreover, the performance of stabilized membranes under practical operating conditions is discussed. Physical aging has been studied extensively in glassy polymers, but aging in CMS membranes has previously focused primarily on adsorption: either chemisorption of oxygen, or physical adsorption of water and organics in the pore structures. Experimentally, in this study, for the samples considered, all of the above adsorption-induced aging mechanisms were excluded as significant factors through thoughtful experimental design. Physical aging appears to be the primary cause for rapid changes of transport properties in early stages after membrane fabrication for samples derived from high fractional free volume precursors. The CMS pores are believed to age analogously to the "unrelaxed free volume" in glassy polymers. Over time, these pores tend to shrink in order to achieve thermodynamically more stable states. Results of sorption tests in CMS also support the above hypothesis. The significance of physical aging phenomena on membrane testing protocols, structural tailoring, and performance evaluation are discussed. A long term permeation test demonstrated excellent stability of stabilized CMS membranes under realistic conditions.