The Mo/Al2O3 catalyst was modified by Ir (0.1-0.8wt.%) and tested in the parallel hydrodenitrogenation (HDN) of pyridine and hydrodesulfurization (HDS) of thiophene. It was found that Ir addition had positive effect on both HDN and HDS, influenced by Ir loading and by the: way of catalyst preparation. A synergetic effect between Mo and Ir was observed in HDN (three times) while not in HDS. The: HDN/HDS selectivity of modified catalysts reached a maximum at 0.34-0.53% Ir, that being higher than selectivities of single Ir and Mo catalysts. Results of H-2 adsorption on the reduced catalysts and temperature-programmed reduction of sulfided catalysts confirm interaction between Ir and Mo, resulting in increased reducibility and activity of modified catalysts. Thr activities of the modified catalysts in HDS and hydrogenation of pyridine correlated with H-2 consumption during the TPR while activity in piperidine hydrogenolysis was parallel to Ir dispersion. Results of H-2 adsorption and transmission electron microscopy suggest that the dispersion of Ir is important for improvement of HDN activity and not its amount.