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Two industrial hydrotreating catalysts, CoMo/AlnOs and NiMo/Ai203, were compared with respect to their activity and selectivity In thiophene hydrodesuiphurisation and cyclohexene hydrogenation at 280°C and 4 MPa. Their difference in catalytic properties for HDS and hydrogenation was found strongly dependent on the experimental conditions. Addition of small amounts of H2S to the feed inhibits the activity of both catalysts; but at high H2S levels, the catalysts remain active. A new type of site Is proposed to operate at high H2S levels. The NiMo catalyst was found far more active than the CoMo catalyst at low H2S pressure, while the two catalysts became equivalent at high H2S levels.-189 -