[C-13]Methanol and [C-12]benzene were coreacted over zeolite H-beta at different reaction temperatures, ranging from 210 to 330 degreesC. The main attention was paid to the material being trapped within the zeolite pores. Hexamethylbenzene and the heptamethylbenzenium cations were easily formed by methylation reactions. The arene methylation reactions leading to hexamethylbenzene and the heptamethylbenzenium cation were much faster than the reactions leading to isotopic scrambling, and the isotopomers C-12(6)((CH3)-C-13)(6) and C-12(6)((CH3)-C-13)(7)(+) could be synthesized in situ. This achievement was used to elucidate new details in the MTH mechanism. The heptamethylbenzenium cation was the major compound being retained within the pores of the beta zeolite at 250 degreesC. When the reaction temperature was increased, this species soon vanished and polymethylbenzenes and -naphthalenes became the dominating components. The isotopic distribution observed in both gaseous products and retained material is in excellent agreement with a reaction mechanism where rearrangement of the heptametylbenzenium cation, followed by dealkylation, is the major reaction route for olefin formation in the MTH/MTO reaction over the H-beta zeolite. (C) 2003 Elsevier Inc. All rights reserved.