In order to investigate the nature of Ni supported on alpha-Al2O3 and its modification with Sn in methane reforming reactions, a theoretical study was carried out. A molecular orbital approach of the extended Huckel type was performed to obtain the formation energies of postulated adsorbed and reacted species of CH4, H2O, O-2 and CO2 on a model of Ni and NiSn surfaces resembling Ni/alpha-Al2O3 and NiSn/alpha-Al2O3. Three different known planes of fcc Ni were considered: (111), (001) and (110). Possible adsorbed and reacted species of CH4, H2O, O-2 and CO2 on Ni with Ni all around, Sn fully rounded by Ni and Ni near a Sn were discussed using reaction energies. The interaction NiSn in Ni clusters has been claimed to be of interstitial substitution-type in alloys. The (111) plane is the most reactive for adsorption. In agreement with experimental work. methane activation on Ni is the controlling step in reforming of methane with CO2 (R), partial oxidation with O-2 (POM) and mixed reforming using CO2 and O-2 (MR). Sn near a Ni changed the adsorption properties of it. Favoured reactions for Sn rounded by Ni on (001) and (111) planes are the adsorption of CO2 and formation of a Sn-CO bond and the generation of Sn-H bonds from methane and water dissociation. The theoretical results are discussed in the context of previously published experimental data. (C) 2003 Elsevier Science B.V. All rights reserved.