The disproportionation of ethylbenzene was investigated over high-silica zeolites prepared by both direct crystallization and dealumination. It was found that dealumination increases the rate of the reaction and improves the life time of the mordenite catalysts. It was shown, however, that the catalytic performance of dealuminated mordenites is not unambiguously related to the number of the Al atoms per unit cell, N-Al(F), remaining in the framework. Rather, it depends on the method of dealumination. Catalysts prepared via heat treatment of NH4-mordenites and subsequent acid leaching proved to be superior to those having the same N-Al(F) but were obtained by acid leaching of Na-mordenite. A mordenite sample prepared through the former method and possessing a Si/Al ratio of, e.g., 11.5 exhibited a rather high activity and catalytic stability compared to materials obtained via the second procedure. The differences in the catalytic performance of mordenites produced through different methods are discussed in terms of microporosity and both density and strength of acidic sites. in particular, it is inferred that the so-called isolated (0-NNN) Al atoms play an important role. The conditions for preparation of mordenites are outlined that are favorable for the enhanced formation of p-diethylbenzene,