The purification of benzene derivatives, particularly xylene isomers, is one of the most important organic mixture separations practiced in industry. The separation of xylene isomers is especially challenging due to the similarity of their physical properties. Carbon molecular sieve (CMS) membranes are promising materials for such challenging solvent separations due to their thermal and chemical stability, but these materials have not been studied in detail in the case of large organic molecules. Xylene isomer transport and sorption properties in a CMS membrane derived from a prototypical polymer of intrinsic microporosity (PIM-1) reveal that diffusion selectivity is the dominant factor in contributing to the preferential permeation of p-xylene over o-xylene. Moreover, the contributions of "enthalpic" and "entropic" selectivity to the diffusion selectivity are studied in detail and reveal that entropic factors dominate the xylene selection mechanism. Overall, this study provides fundamental insight and guidance into the separation of large organic molecules in amorphous microporous materials.