Molybdenum oxynitride was prepared by hydrazine reduction of MoO3 at moderate temperatures. The anhydrous condition was favorable to production of amorphous molybdenum oxynitride, and the presence of hydrogen favored the reduction of Mo6+ and Mo4+ species to Modelta+ (0 < δ < 4) species. These molybdenum oxynitrides exhibited activity for hydrogenation which depended on the amount of Modelta+ ((0) < δ < 4) species produced under reaction conditions. The amorphous molybdenum oxynitride MoO1.83N0.36 catalyst showed a good catalytic activity, selectivity, and resistance to poisoning of H2S for liquid-phase hydrogenation of longer-chain alkadienes.