The surface species formed from the adsorption of 1,3-butadiene and l-butene on Mo2N/gamma -Al2O3 catalyst have been characterized by in situ FT-IR spectroscopy with an attempt to get insight into the selective hydrogenation of 1,3-butadiene on transition metal nitride catalysts. 1,3-Butadiene adsorption on the Mo2N/ gamma -Al2O3 catalyst forms several kinds of chemisorbed surface species: pi -adsorbed butadiene (pi (s) and pi (d)); sigma -bonded species; dehydrogenated species. pi (s)-and sigma -bonded butene species are present on the surface for l-butene adsorption and reaction on the nitride catalyst. The presence of the pi (d)-adsorbed butadiene significantly prevents I-butene from its adsorption, further hydrogenation, and isomerization on the nitrided catalyst, accordingly resulting in high selectivity in the hydrogenation of 1,3-butadiene to l-butene. The surface nitrogen atoms weaken the interaction between 1,3-butadiene and the catalyst surface, and result in the high selectivity in the hydrogenation of 1,3-butadiene to 1-butene.