The steam reforming of ethanol/phenol mixture (168 g(ToT)/Nm(3), ethanol: phenol 2:1 mol), assumed as a model for tar mixtures, has been studied over Ni/Al2O3, Ni-Mg/Al2O3 and Ni-B/Al2O3 catalysts. The effects of reaction temperature and of the presence of sulphur have been investigated. Catalysts have been characterized by XRD, IR of adsorbed CO, TPR, UV-vis spectroscopies. IR studies of adsorbed water and phenol have also been performed. The addition of Magnesium shows an activating effect on Ni/Al2O3 but also increases sensitivity to sulphur poisoning. This has been associated to the formation of large Ni metal particles over the MgxAl2O3+x support surface formed by the penetration of Mg ions in the subsurface of alumina, and to the low activity of the surface of such a modified support in the conversion of the organic reactants. A possible role of Mg in a different, possibly more efficient, activation of water is envisaged. The addition of boron in small amounts reduces activity, mainly if boron is added to the support before impregnation of Ni. A slight positive effect in reducing the deactivation due to sulphur is observed when boron is added after nickel.