Abstract The surface properties and catalytic activity of nickel powders impregnated with n-alkanethiols (H–(CH 2) 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–C 4S and Ni–C 5S catalysts were similar to those of pristine Ni catalysts. However, the activity of Ni–C 6S catalysts decreased for temperatures above ∼580 °C and no activity was obtained over the Ni–C 10S 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 ∼4.6%.