A molybdenum oxycarbide catalyst that is very active and selective for the isomerization of n-alkanes has been characterized by means of diffraction techniques and simulations. The proposed structure is confirmed by transmission electron microscopy. An elemental analysis performed on the pure phase prepared from a bronze precursor provides the stoichiometry of this new catalyst. Quantitative temperature-programmed oxidation measurements confirm the elemental composition. The stoichiometry of this new phase, MoO2.42C0.23H0.78, shows an excess of light elements O, C, and H only possible because of the presence of metal vacancies. A tentative explanation of its peculiar catalytic properties is deduced from these structural and chemical features. (C) 2000 Academic Press.