This study examines the influence of catalyst activation for methane decomposition over Co-Al mixed oxides derived from hydrotalcites. Samples were prepared by coprecipitation and characterized by surface area measurements and temperature-programmed reduction. Spent catalysts and carbon produced in the reaction were characterized by X-ray diffractometry, temperature-programmed oxidation and scanning electron microscopy. Activity runs using previously reduced samples with H-2 or activated under CH4 flow were carried out in a fixed-bed reactor between 500 and 750 degrees C using in-line gas chromatography analysis. The specific surface area decreases as the Co/Al ratio increases, which is related to the increased Co3O4 phase rather than Co-Al mixed oxides. The TPR results indicate the reduction of four types of Co species: Co3+ and Co2+ species from Co3O4, and Co from inverse spinel (Co2AlO4) and normal spinel (CoAl2O4). Reduction with hydrogen at 750 degrees C was very severe. Samples reduced with H-2 showed large Co degrees crystallites, which increased with the Co/Al ratio. Co-Al catalyst activation under methane flow leads to lower crystallite size and higher thermal stability for hydrogen production by methane decomposition. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.