The deactivation of Ni/SiO2 catalysts by thiophene or H2S during benzene hydrogenation at 200 degrees C was studied as a function of the nickel reduction degree. An initial toxicity higher for thiophene than for H2S was found at this relatively high temperature. Furthermore, the thiophene/H2S toxicity ratio decreases on increasing the reduction degree of the sample. Temperature-programmed surface reaction (TPSR) results suggest that the difference in toxicity may be explained through a carbonaceous layer that remains adsorbed at 200 degrees C on the nickel surface. TPSR results also revealed that thiophene is hydrogenolyzed to butane to a considerable extent on nickel surfaces at room temperature.