Oxynitrides of early transition metals are bifunctional catalysts active in hydrodenitrogenation (HDN). The strength of the hydrogenating function and the acidic properties of molybdenum and niobium oxynitrides were investigated, as well as the inhibiting effect of ammonia and the effect of solvent (cyclohexane or tetradecane) on HDN of 1,2,3,4-tetrahydroquinoline (1-4 THQ). Even under high hydrogen pressure (4.5 MPa) the reaction was found to be able to proceed without hydrogenation of the aromatic cycle of 1-4 THQ before C-N bond scission, which is not the case over sulfided NiMo supported catalysts in HDN. Experiments in presence of ammonia permitted to support a bifunctional dual site concept.