The direct catalytic conversion of chloromethane to higher hydrocarbons at various temperatures over a series of zeolites ZSM-5 exchanged with alkali cations has been studied by means of in situ FTIR and catalytic testing. Hydrocarbons such as aromatics, alkanes and alkenes are formed. A correlation between the acido-basic properties of ZSM-5 zeolites exchanged with alkali cations (Li, Na, K and Cs) and their activity in the direct catalytic conversion of chloromethane has been established. The reaction mechanism has been discussed. The IR observations indicate a production of HCl by elimination from CH3Cl during the first step of the conversion. A co-operative role of counter ions and the framework oxygen atoms is responsible for the conversion of CH3Cl in cationic zeolites. Coke fort-nation during the reaction has been investigated by thermal analysis. Significant differences in activity and selectivity can be observed between Li- and NaZSM-5 on one side and K- and CsZSM-5 on the other side. The conversion of chloromethane is quite higher on Li- and NaZSM-5. On the Li and Na forms, the selectivities in C5-C6 are higher. While on K- and CsZSM-5 zeolites the C4 unsaturated products are favoured. (C) 2002 Elsevier Science B.V. All rights reserved.