The idealized mordenite pore structure comprises an array of parallel tubes only slightly greater in diameter than the minimum effective diameter of benzene and cumene molecules, and they cannot readily counterdiffuse in mordenite. The cumene desorptive diffusion coefficient from H-mordenite into benzene at 25°C decreased by over two orders of magnitude as the time of saturation before desorption was increased to 6 days. This decrease seems to be caused by the slow formation in the H-mordenite pores during the saturation step of radical ions and diisopropylbenzene molecules, the latter by the disproportionation of cumene. Both of these cause blockage of the pores. Most of the surface area of H-mordenite is apparently unavailable for reaction of aromatic species at temperatures moderately above ambient.