The catalytic properties of alumina-supported Ir catalysts (approximate to 1 wt% Ir) were studied in the hydrodenitrogenation (HDN) of pyridine at 320 degrees C and 20 bar of pressure in the absence as well as presence of parallel hydrodesulfurization (HDS) of thiophene. The effects of Ir precursor (Ir(AcAc)(3), Ir-4(CO)(12), H2IrCl6, (NH4)(2)IrCl6),metal dispersion and sulfur addition were investigated. Ir-4(CO)(12) gave the most active catalyst which was ascribed to a lower amount of contaminants originated from the starting Ir compounds rather than to a better Ir dispersion. The decrease of Ir dispersion by sintering in air led to much higher decrease of the rate of C-N bond hydrogenolysis than that of pyridine hydrogenation. The Ir dispersion determined partly the HDN selectivity; a better dispersed Ir phase gave a lower amount of intermediate piperidine. Presulfidation of the reduced catalyst led to 20% decline of the rates of both consecutive HDN steps. An additional and much larger activity decline was caused by the simultaneous execution of HDS. The competitive adsorption of thiophene (or H2S) was selectively affecting C-N bond hydrogenolysis more than pyridine hydrogenation. The alumina-supported Ir catalysts possessed much higher HDN activity and HDN/HDS selectivity than a conventional NiMo system.