Oxynitrides of early transition metals are bifunctional catalysts (metallic and acidic sites) active in hydrodenitrogenation (HDN). The HDN of indole was used as a molecular probe reaction to study the metallic and acidic properties of the dual sites on oxynitrides. The behavior of those sites was found to be different from that of supported and sulfided NiMo. The reaction was conducted at low partial pressure of H2S over both a bulk MoOxNy and a supported NiMo catalyst, nitrided before the reaction. Under high hydrogen pressure, HDN of indole was shown to occur with or without prior hydrogenation of the aromatic ring of orthoethylaniline, leading to either ethylbenzene or ethylcyclohexane. Secondary reactions, such as the joining of heterocyclic rings (leading to 1-4 tetrahydroquinoline) and dimerization, were found to occur as a result of the presence of acidic sites. Hydrogenolysis of the lateral chain of orthoethylaniline was also observed.