Oxidic and reduced and passivated Al2O3-supported nickel catalysts with loading up to 19.3 wt% were characterized by temperature-programmed sulfiding (TPS), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy. The oxidic catalyst precursors contain a highly disperse nickel species consisting of either an oxidic core and a hydroxide outer layer or a hydroxide as a whole, dependent on the Ni loading. The crystallite size of the active phase varies between 0.4 and 1.6 mn. This high dispersion was maintained during sulfidation. From TPR and TPS, it is inferred that no nickel aluminate is present. During reduction and passivation the crystallite size increases to 1.3-2.5 mn. From TPR of the oxidic catalyst it was concluded that the active phase contained more oxygen than that corresponding to the stoichiometry of NiO since an excess of hydrogen of 20-50% was consumed for the removal of the reactive oxygen species.