The transformation of dichloromethane (DCM) was investigated over NaX and NaY zeolites under the following conditions: fixed-bed reactor, 1000 ppm of DCM in wet air, space velocity of 20,000 h(-1), temperature between 220 and 450degreesC. At short time on stream, there is, with both zeolites, a stoichiometric reaction of DCM and water with the ONa groups, an oxidation of the formaldehyde product, and a significant change in the zeolite physicochemical properties: elimination of the strongest basic sites, creation of OH groups, deposit of NaCl species, etc. After a significant decrease in conversion, DCM transforms catalytically through hydrolysis into HCl and formaldehyde. However, very large differences can be observed between the zeolite samples: higher activity of NaX especially at short times on stream, more pronounced change in its physicochemical properties, etc. Most of these differences can be related to the higher basic strength of the NaX zeolite. (C) 2003 Elsevier Inc. All rights reserved.