Selectivity of hydrodenitrogenation/hydrodesulfurization (HDN/HDS) using carbon supported Ni, Mo, and Ni-Mo sulfide catalysts was studied at a pressure of 2 MPa and over a temperature range of 280-350 °C. A commercial alumina supported Ni-Mo sample was also included as reference catalyst. Model compounds used were pyridine and thiophene. Selectivity (HDN/HDS) increased with decreasing temperature and decreased in the order Ni, Mo, Ni-Mo. Ni/C catalyst exhibited unusually high HDN/HDS selectivity at low temperature, where HDN of pyridine was faster than HDS of thiophene. Selectivity was interpreted as an intensive property of a catalyst characterizing quality of active surface. The combination of Ni and Mo sulfides in Ni-Mo catalyst resulted in a strong shift in HDN/HDS selectivity to the HDS side; the selectivity of Ni-Mo sample was very low and was not an average of the selectivities of Ni and Mo catalyst. This was interpreted as the evidence of the chemical synergism in the Ni-Mo sulfide system, because structure synergism cannot cause such selectivity changes accompanying combination of Ni and Mo sulfide into the mixed catalyst.