Simultaneous catalytic hydrodesulfurization(HDS) of thiophene and hydrodenitrogenation(HDN) of pyridine were studied over sulfided CoMo/Υ-Al₂O₃ catalysts and this study was carried over the range from 423K to 723K, from 10×10⁵Pa to 50×50⁵Pa and from 0.015g-cat.hr/ml-feed to 0.03g-cat.hr/ml-feed of temperature, pressure and contact time, respectively. Pyridine HDN was more diffuclt than thiophene HDS and thiophene HDS was inhibited by the presence of pyridine at all temperatures and pressures studied. But thiophene had a dual effect on HDN. At low temperatures pyridine HDN was inhibited by the presence of thiophene, but at high temperatures it was enhanced by the thiophene. The formation of piperidine was greatly decreed with the addition of thiophene at all temperatures and pressures. CoMo/Υ-Al₂O₃ showed better activity then NiMo/Υ-Al₂O₃ for the pure thiophene HDS but NiMo/Υ-Al₂O₃ showed greater activity than CoMo/Υ-Al₂O₃ for pure pyridine as well as that of mix-ed feed of thiophene and pyridine.