The paper presents an investigation on the catalytic activity for the ethanol steam reforming of Co3O4 oxidized, reduced and supported on MgO, and of CoO in MgO solid solution. Only samples containing metallic cobalt are found to be active for reforming reaction. H-2-TPR characterization of aged samples shows that reaction mixture oxidizes a small fraction of metallic cobalt to Co+2. A distinct role of Co+2 and Co-0 in the reaction is enlightened.