Carbon molecular sieve (CMS) membranes with attractive separation properties were formed by pyrolysis of a polymeric precursor. Defect-free membranes with oxygen/nitrogen permselectivities three times greater than the polymer precursor were obtained. Gas separation properties were also measured at intermediate stages during the pyrolysis protocol to study the evolution of entropic selectivity, which distinguishes molecular sieving materials from typical polymeric materials. Initially, permeabilities increase dramatically during the pyrolysis process due to an increase in overall sorption coefficients. In the finally pyrolyzed membrane, however, permeabilities are three times lower than in the polymer precursor due to significantly lower oxygen diffusion coefficients. Nevertheless, the separation properties of the pyrolyzed membranes are well above the so-called property 'upper-bound trade-off curve' often used to compare conventional polymeric materials. The increase in permselectivity is entirely due to an increase in mobility selectivity. Entropic selectivity increases are responsible for the higher mobility selectivity in the finally pyrolyzed membranes; however, energetic contributions were more significant for materials at the intermediate stage. Significant conclusions about the structure of the evolving molecular matrix can be drawn from the gas separation results.