An investigation of the effect of sulfur on the activity of a bulk molybdenum carbide catalyst for the steam and oxidative-steam reforming of tri-methyl pentane, has been under-taken. Using thiophene as a model sulfur compound at concentrations as high as 1000 ppmw, the degree of deactivation was found to be dependent on the sulfur concentration but was minimal at concentrations below 100 ppmw. While deactivation was completely reversible in the case of steam reforming, spent catalysts from oxidative-steam reforming could only be partially reactivated. Characterization of spent catalysts by XRD and XPS leads to the conclusion that deactivation during steam reforming is due to surface oxidation of the carbide, apparently as a result of sulfur inhibition of carburizing kinetics. On the other hand, deactivation during oxidative steam reforming appears to be due to coking, attributed to increased surface acidity resulting from SO, species. (c) 2005 Elsevier B.V. All rights reserved.