The surface properties and catalytic activity of nickel powders impregnated with n-alkanethiols (H-(CH2)(n) -SH, n=4, 5, 6, and 10) for the steam reforming of methane have been studied. The quantity and state of carbon and sulfur on fresh and used thiol-contaminated Ni catalysts were determined using XPS. The high catalytic activity and stability of Ni-C4S and Ni-C5S catalysts were similar to those of pristine Ni catalysts. However, the activity of Ni-C6S catalysts decreased for temperatures above similar to 580 C and no activity was obtained over the Ni-C10S at any temperature. Catalytic deactivation was principally caused by high surface coverages of aromatic carbon. The surface aromatic-carbon was due to the pyrolysis of the pre-adsorbed n-alkanethiols and not from the feed-gas methane. The aromatic carbon leads to the deactivation of the catalyst for the steam reforming of methane at coverages above similar to 4.6%. (c) 2006 Elsevier B.V. All rights reserved.