Ligand-free Ru nanoclusters supported on carbon black have been synthesized in situ for the first time from the reduction of RuCl3 by ammonia-borane concomitantly with its hydrolysis process at room temperature, and their catalytic activity has been investigated. Well dispersed Ru nanoclusters (similar to 1.7 nm) are stabilized and immobilized by carbon black. Due to the small size and the absence of ligands on the surface, the Ru catalysts exhibit high catalytic activity, which is partly retained after 5 reaction cycles. A kinetic study shows that the catalytic hydrolysis of ammonia-borane is first order with respect to Ru catalyst concentration; the turnover frequency is 429.5 mol H-2 min(-1) mol(-1) Ru. The activation energy for the hydrolysis of ammonia-borane in the presence of Ru/C catalysts has been measured to be 34.81 +/- 0.12 kJ mol(-1), which is smaller than most of the values reported for other catalysts, including those based on Ru, for the same reaction. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.