The fluxes of aromatic molecules (p-xylene, o-xylene, and benzene) were measured as a function of temperature and feed partial pressure through several molecular sieve membranes (SAPO-5, SAPO-11, and mordenite) and three types of MFI membranes (silicalite-1, ZSM-5, and boron-substituted ZSM-5). Single-file diffusion appeared to control transport through the SAPO and mordenite membranes. Hence, those membranes showed ideal selectivities greater than 1 for benzene over the xylene isomers but no separation selectivities for the mixtures. Surface diffusion and activated gaseous transport were the controlling mechanisms for the MFI membranes. The highest p-xylene/o-xylene selectivities were obtained for a boron-substituted ZSM-5 membrane. At feed partial pressures of 2.1 kPa and at a temperature of 425 K, the best selectivities were 130 (ideal) and 60 (separation). Zeolite pores preferentially permeated p-xylene and took as long as 8 h to reach steady state. Nonzeolite pores preferentially permeated o-xylene after much shorter breakthrough times. Higher pressures of p-xylene distorted the membrane framework, resulting in increased o-xylene permeation and reduced selectivity. After reaching steady state, the flux of p-xylene through zeolite pores was stable for at least 10 h. The flux of o-xylene through nonzeolite pores was similarly stable at 373 K but continuously decreased for at least 12 h at 405 K.