The conversion of chloromethane to light olefins, like ethene and propene, was studied over ZSM-5 catalysts at 698 K. For HZSM-5, the selectivity to olefins increased for higher Si/Al ratio and was interpreted in terms of reducing hydrogen transfer, as the sites become more isolated. The exchange of Zn2+ cation increased the activity and selectivity to light oletins. Theoretical calculations using a MT6 cluster (T = Si or Al and M is an alkaline cation) showed that the activation energy for the formation of an adsorbed methoxide from chloromethane increases in the alkaline metal series, with Li+ showing the lowest and Cs+ the higher barrier. The calculations also indicated that the metal-chloride interaction plays an important role in the reaction. © 2004 Elsevier B.V. All rights reserved.