Ni based catalysts supported over gamma-Al2O3, previously modified with increasing contents of Mg(II), were employed for the steam reforming of glycerol. The aim of the present study is to analyze the effect of the content of Mg(II), as a promoter of Ni/gamma-Al2O3 catalysts, on the textural and structural characteristics of the solid; as well as on the catalytic activity and selectivity to H-2 in the steam reforming of glycerol. Fresh samples were characterized by PXRD, BET surface area, H-2 chemisorption, TPR, and CO2-TPD. Used catalysts were analyzed by TPO, in order to study the effect of Mg(II) on carbon gasification. Both fresh and used samples were examined by SEM. The content of Mg(II) has both an effect on the catalytic performance and on the structural and textural characteristics of the catalysts. The incorporation of Mg(II) results in the formation of Mg1-xAl2O4-x spinel phase. The differences in catalytic properties due to the increasing content of Mg(II) have effect simultaneously on the Ni degrees crystallite size, on the acidic-basic character and on the interactions between NiO and support. For the catalysts promoted with Mg(II), the best activity for the steam reforming of glycerol was achieved with Ni(10)Mg(3)Al catalyst while the Ni(10)Mg(15)Al catalyst formed the lowest amount of carbon during reaction time on stream. The catalyst prepared without Mg(II) presented good activity results despite the lowest Ni dispersion. This behaviour was assigned to the presence of Ni sites more active for the steam reforming of glycerol than the ones on the catalyst promoted with Mg(II). However, this catalyst had the highest carbon deposition during reaction time on stream. High contents of Mg(II) inhibited carbon formation, this was evidenced by TPO analyses performed to used samples. Low carbon formation at high Mg(II) could be related to the higher basic character of the support as the content of Mg(II) increases. (C) 2013 Elsevier B. V. All rights reserved.