In this work, the deactivation of a Ni/SiO2 catalyst by thiophene and H2S during benzene hydrogenation at 200 degrees C was studied. An initial toxicity approximately three times higher for thiophene than for H2S was found. Temperature-programmed surface reaction (TPSR) results of previously adsorbed thiophene suggest that this difference in toxicity may be explained in terms of incomplete thiophene hydrogenolysis at 200 degrees C, TPSR results have also revealed that thiophene is hydrogenolized to a considerable extent on nickel at room temperature.