An improved transient (CH3OH)-C-12/(CH3OH)-C-13 switching method is used at steady-state conditions with a computer-controlled multi-position valve to distinguish the formation paths of propene and ethene over HZSM-5 zeolites in the methanol-to-olefin process for understanding the effects of Si/Al ratios and the carbonaceous deposited in the zeolites. By carefully analyzing the isotope distribution in products in short time intervals at transient state, it is demonstrated that a fraction of the carbonaceous species is active and participates in the methanol reactions. On high silica zeolite, the active carbonaceous species are responsible for the production of ethene, while propene is formed mainly through rapid alkene methylation in zeolite channels. High Si/Al ratio of zeolite leads to high ratio of propene to ethene in products of methanol-to-olefin process. (C) 2012 Elsevier B.V. All rights reserved.