The reactivity of a series of amines with various structures and different numbers of hydrogen atoms on the carbon atoms in the alpha and beta position, with respect to the nitrogen atom;, was examined on four transition metal sulfides, i.e. NbS3, MoS2, RuS2, and Rh2S3, II is shown that the reaction mechanism proceeds via an elimination or a nucIeophilic substitution the relative importance of which depends on the structure of the substrate to be transformed and on the transition metal sulfides properties. NbS3 is the most active sulfide of the series for the elimination reaction due to its high acidity, but it is inactive for the nucleophilic substitution. On the other hand, the surface species of Rh2S3;can be involved in a nucleophilic substitution but not in an elimination reaction, The other sulfides of the series behave in between. These results clearly demonstrate that the catalysts intervene differently in the HDN mechanism. Moreover, for a given solid the structure of a nitrogen-containing; molecule strongly affects the elementary steps of its transformation, Accordingly, a precise mechanistic study of the reactivity of a mode! molecule al the surface of a sulfide cannot be generalized to the overall HDN process which involves several types of molecules.