The regeneration of an n-decanethiol-poisoned nickel catalyst by treatment with steam was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoemission spectroscopy (XPS) and mass spectrometry (MS). The catalytic activity of a Ni catalyst contaminated with n-decanethiol (H-(CH2)(10)-SH) before and after regeneration was measured on methane steam reforming at a CH4:H2O ratio of 1:2 at 700 degrees C and compared to the nickel catalyst. The results show that, although (a) in the regenerated catalyst surface there is no residual sulfur and (b) the amount of surface carbon is the same as in Ni catalyst, the regenerated catalyst has gained 84% of its catalytic activity. Since all sulfur is removed, this loss of activity is attributed to structural changes at the surface of the catalyst and to the different nature of the remaining carbon. (C) 2007 Elsevier B.V. All rights reserved.