The hydrodenitrogenation of 2,6-dimethylaniline (DMA) was studied over a series of sulfided Co(0-4.7%)-Mo(8.7%)/Al2O3 catalysts at 573 K under 4 MPa total pressure and 0-56 kPa H2S partial pressure. Two NiMo samples were tested for comparison. The reaction network presents three parallel routes: dearomatization of DMA followed by either hydrogenation-elimination to dimethylcyclohexenes and dimethylcyclohexanes, or NH3 elimination to m-xylene, and disproportionation of DMA to 2-methylaniline and 2,4,6-trimethylaniline. We demonstrate that part of the xylene is formed by direct aromatic carbon-nitrogen bond cleavage through a nucleophilic substitution involving hydride species. On CoMo catalysts in the presence of H2S, the amount of extra xylene is independent of Co content, while the dearomatization is promoted. Without H2S, this special substitution reaction is most important on the Mo catalyst, and strikingly Co acts as a poison. FT-IR spectroscopy of adsorbed carbon monoxide evidences a new type of sites on the sulfided catalysts after a mild hydrogen treatment. We propose that a site configuration located exclusively on unpromoted Mo atoms highly depleted in sulfur is responsible for the direct denitrogenation route. The NiMo couple behaves differently: xylene formation is independent of Ni content, which means that the specific Mo sites for direct C-N bond rupture are poisoned by nickel, even in the presence of H2S. The location of Co and Ni on the MoS2 slabs then appears different.