Carbided molybdenum catalysts supported on ultrastable Y zeolites were prepared by adsorption of molybdenum hexacarbonyl vapors at 343 K or wet point impregnation with ammonium heptamolybdate solutions, followed in both cases by temperature-programmed carburization under a 20% methane in hydrogen mixture up to 923 K. Nitrogen physical adsorption and X-ray diffraction clearly established that the zeolite structure was preserved during the carburation treatment. Chemical analysis of the catalysts evidenced that very little loss of molybdenum occurred. Despite the fact that all catalysts had similar CO chemisorption capacities, their activity varied widely. For the same silica-alumina ratio (SAR), activity increased with increasing sodium content and, for low sodium contents, with increasing SAR. When catalyst reduction was performed with pure hydrogen, the activity was about one third of the one obtained with the carburation procedure, suggesting that a carbon containing species is the active one in this reaction. Two EPR signals with g(perpendicular to) values 1.96 and 1.98 were observed in the carburized catalysts, that may be attributed to hexa-coordinated Mo5+. The second one of these signals only appears in the most active catalysts and is attributed to Mo5+ ions in small carbide or oxycarbide clusters. (C) 2004 Elsevier B.V. All rights reserved.