The reactions of 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene (durene), pentamethylbenzene, hexamethylbenzene (HMB), ethylbenzene, and cumene were studied on large-pore zeolite HBeta catalysts, either alone or with co-injection of methanol-C-13. The reactivity of the methylbenzenes alone increased with increasing methyl substitution, as did selectivity for propene over ethylene. Disproportionation occurred for all methylbenzenes studied, in the case of HMB, pentamethylbenzene was a major volatile product, and we inferred that the heptamethylbenzenium cation also formed and remained in the catalyst. Substantially higher yields of olefins were obtained when methylbenzenes were co-reacted with methanol-C-13. Ethylbenzene alone was unreactive at 350 degreesC. but when injected with five equivalents of methanol-13C, ethylbenzene formed ethylene-C-12(2) with very high selectivity. These and other experiments led to a detailed description of the hydrocarbon pool mechanism for MTO chemistry with side-chain methylation as the predominant route to olefins and the paring reaction as a possible minor pathway.