Glycerol steam reforming, using Mg doped Ni/Al2O3 catalysts, was investigated with the aim to efficiently produce hydrogen. Catalyst containing different Mg loading were prepared by impregnation method maintaining constant the Ni loading (10 wt.%). Catalytic experiments were performed in fixed bed reactor operated at 500 and 600 degrees C. Characterization results revealed that Mg, further to infer a basic character to the carrier, promotes the Ni dispersion. In addition to observe that the catalytic activity was mainly dependent upon the Ni dispersion, the amount and morphology of coke formed during reaction was affected by Mg loading. In particular, the addition of 3wt% of Mg contributes to reduce the coke formation rate while a subsequent addition of Mg up to 10wt%, does not significantly affect neither the catalytic activity nor the carbon formation. By operating at 600 degrees C and with H2O/Gly ratio of 6, low amount of coke, mainly of filamentous nature, was formed. The size and structure of carbon filaments changed as a function of Ni particle size and Mg loading. (C) Copyright 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.