Hexamethylbenzene was reacted over zeolite H-beta at 325 degreesC. The products were C-1-C-5 hydrocarbons, tetra- and pentamethylbenzene. Deactivation was fast. The main attention has been paid to the material retained in the catalyst as deactivation proceeds. The composition of the retained material was determined by dissolving the catalyst in 15% HF and analyzing a CCl4 extract. Methylated benzenes, dihydrotrimethyltraphthalenes, and hexamethyinaphthalene were found. Naphthalenes were present even at the shortest times on stream, showing that hexamethylbenzene alone is able to form coke species as otherwise observed in reactions involving methylbenzenes, methanol, or olefins. Additional experiments involving [C-13]methanol and naphthalene derivatives (either formed in situ or fed separately) gave new insight into the early steps of coke formation. The lowest naphthalene derivative, dihydro-trimethylnaphthalene, is formed from the heptamethylbenzenium ion by a molecular rearrangement and hydrogen transfers. The higher naphthalene derivatives are all formed from dihydro-trimethylnaphthalene by subsequent methylations and hydrogen transfers. Primary alkenes act as hydrogen acceptors. It appears that the main route for deactivation in acid systems where hexamethylbenzene formation takes place is understood. (C) 2003 Elsevier Science (USA). All rights reserved.