Hydrogen reduction of model steam reforming catalysts, comprising about 10wt.% NiO on alpha-Al2O3 with and without the addition of 1-2 wt.% CaO or MgO, was studied with in situ hot-stage X-ray diffraction in the temperature range 175-900degreesC. This technique has the ability to measure NiO disappearance and Ni appearance simultaneously, together with the crystallite size of each. Since the sample was a 50 mum slab of dispersed 20 mum diameter particles, textural and morphological features normally encountered during studies with fixed beds of NiO particles were absent and measurements reflected only the chemical mechanism and kinetics. The rates of disappearance of NiO and appearance of metallic Ni matched each other, with no time lag or induction period. The reaction followed pseudo-first-order kinetics with an apparent activation energy that increased from 95 kJ mol(-1) for the catalyst with no additive to 122 and 322 kJ mol(-1) with added CaO and MgO, respectively. When 2.2 x 10(-2) atm of H2O was added to the reducing gas, activation energies increased by another 23 and 96 kJ mol(-1) for the first two catalysts but showed little effect for the third. There was no indication of any phase other than NiO and alpha-Al2O3 and the lattice constants of NiO remain unchanged. Complimentary scanning electron microscopy (SEM) showed additive oxides as dispersed phases, concentrated in the vicinity of NiO grains. Upon reduction, the metallic Ni crystallites were smaller than the original NiO and were dispersed over a wider range of the surface, suggesting that Ni atoms released from the NiO migrated from the center of reduction. When the reduced catalyst containing MgO was re-oxidized, the ease of reduction approached that of the catalyst without additives, i.e. the reduction process was no longer inhibited by the presence MgO. These observations support a proposed mechanism in which water, adsorbed on the additive oxide in the form of hydrogen-bonded molecules or hydroxyl groups, inhibits the nucleation of Ni atoms, either at the site or in the vicinity, and the difficulty of reduction increases with the hydrophilic nature of the additive. This inhibition is removed following re-oxidation of the catalyst. (C) 2004 Elsevier B.V. All rights reserved.