The influence of alkali-doping of a Ni/SiO2 catalysts on the d-activation by thiophene and H2S during benzene hydrogenation at 200 degrees C has been studied. An initial toxicity higher for thiophene than for H2S has been found for all the catalyst tested. Furthermore, in comparison with the undoped catalyst, the thiophene/H2S toxicity ratio decreases on alkali-doped catalyst. On the basis of benzene hydrogenation active sites, alkali-doping decreases both thiophene and H2S initial toxicity, Temperature programmed surface reaction (TPSR) results suggest that the difference in toxicity may be explained by a layer of hydrocarbon species remaining on the nickel surface after thiophene adsorption at 200 degrees C.