Reactions of C-13-labeled alkanes show that chain growth and cyclization reactions on H-ZSM5 require the initial formation of the corresponding alkene and its extensive participation in rapid oligomerization/beta-scission reactions before cyclization occurs. The role of alkene intermediates was established by the initial formation of predominantly unlabeled products from mixtures of propene and propane-2-C-13 reactants. Aromatic products of propane-2-C-13 reactions on H-ZSM5 contain similar fractions of C-13-atoms and binomial isotopomer distributions. Sequential formation, rapid intramolecular isomerization, and beta-scission reactions of long surface chains must occur during each aromatization turnover in order to form such binomial C-13 isotopomer distributions.