The effects of alkali addition (e.g. Li, Na, K) on the behavior of Ni/MgO catalyst in the bio-ethanol steam reforming have been investigated. Li and Na promote the NiO reduction but negatively affect the dispersion of the Ni/MgO catalyst, whereas K does not significantly affect either morphology or dispersion. Li and K enhance the stability of Ni/MgO mainly by depressing Ni sintering. Coke formation on bare and doped catalysts occurs but with orders of magnitude lower rates than those claimed for Ni supported on an acidic carrier. The peculiar influence of the mean Ni particle size on the turnover frequency (TOF s(-1)) has been explained by invoking a structure-sensitive character of the ethanol dehydrogenation reaction considered as being the first step of the reaction which evolves according to the following mechanism: ethanol hydrogenation --> acetaldehyde decomposition --> steam reforming of methane and water gas shift reactions. (C) 2004 Elsevier B.V. All rights reserved.