The surface regeneration or de-sulfurization process of a sulfur-poisoned (i.e. sulfur-covered) nickel surface by O-2 and H2O has been studied using first-principles calculations with proper thermodynamic corrections. While O-2 is more effective than H2O in removing the sulfur atoms adsorbed on nickel surface, it readily reacts with the regenerated Ni surface, leading to over-oxidization of Ni. Thus, H2O appears to be a better choice for the surface regeneration process. In reality, however, both O-2 and H2O may be present under fuel cell operating conditions. Accordingly, the effects of the partial pressures of O-2 [p(O2),] and H2O [p(H2O)] as well as the ratio of P-O2/p(H2O) on the regeneration of a sulfur-covered Ni surface without over-oxidization at different temperatures are systematically examined to identify the best conditions for regeneration of Ni-based SOFC anodes under practical conditions. (c) 2007 Elsevier B.V. All rights reserved.