The NaBH4 hydrolysis over Ru catalysts on different supports was here investigated. It was found that the support has a strong effect on the catalytic activity which was found to be in the order: Ru/activated carbon >> Ru/CeO2 > Ru/TiO2 > Ru/gamma-Al2O3. On the most active system, namely Ru/activated carbon, the effect of the Ru loading and the metal precursor (RuCl3 and Ru(NO)(NO3)(3)) was studied in more detail. Tests carried out with and without temperature control demonstrated that, for each precursor, the activity increases on increasing the Ru content. Moreover samples prepared by RuCl3 were more active than the corresponding ones prepared by Ru(NO)(NO3)(3). Characterization data, namely CO chemisorption and TEM analysis, point out that the Ru particles size is strongly affected from the Ru precursor used, Ru(No)(NO3)(3) resulting in very small Ru particles (diameter < 1 nm), RuCl3 leading to Ru nanoparticles with average diameter of 2.4 nm. This latter size was found to be appropriate for the NaBH4 hydrolysis reaction, which is proposed to be a structure sensitive reaction. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.