We investigated the difference between the mechanisms of methanol conversion and ethanol conversion to propylene over ZSM-5 catalysts. The mechanism of methanol conversion depended on the Si/Al-2 ratio, whereas the mechanism of ethanol conversion did not. We believe that at a high Si/Al-2 ratio, propylene was produced from methanol via dimethyl ether by means of the hydrocarbon pool mechanism. In contrast, at a low Si/Al-2 ratio, dimethyl ether was dehydrated to ethylene, which was then directly converted to propylene. In the case of ethanol conversion, propylene was produced from ethylene, which was formed by intramolecular dehydration. We propose that ethylene was transformed to a carbene species, which was a transient intermediate for direct production of propylene. (C) 2013 Elsevier B.V. All rights reserved.