Cobalt catalysts were prepared by wet impregnation of four distinct supports: alumina, magnesia and two mixed supports of alumina with magnesia (10 and 30 wt%), prepared by wet impregnation of magnesia precursor on alumina. Magnesia addition decreased catalyst acidity and reduced spinel phase formation; however, high magnesia content (30 wt%) decreased catalyst reducibility and cobalt dispersion. The catalysts were evaluated on steam reforming of glycerol for 30 h at 500 degrees C, GHSV of 200,000 h(-1) and a glycerol solution 20% v/v in the feed. All catalysts, except the catalyst supported on pure magnesia, presented deactivation during reaction time, because of coke formation and reoxidation of metallic cobalt phase, mainly for the catalyst supported on 30% MgO-Al2O3. The catalyst supported on Al2O3 exhibited the highest mean conversion to gas (42.4%) and hydrogen yield (31.2%) during time on stream. However, the catalyst supported on 10% MgO-Al2O3 presented higher stability in terms of glycerol conversion to gas and hydrogen yield, associated with preferential formation of filamentous coke instead of amorphous coke. [GRAPHICS] .