To our knowledge, the present study is the first investigation by liquid-phase calorimetry of the mechanism of hydrogen generation by hydrolysis of sodium borohydride catalyzed by Co2B nanoparticles generated in situ. The differential reaction calorimeter was coupled with a volumetric hydrogen measurement, allowing a simultaneous thermodynamic and kinetic study of the reaction. At the end of the reaction, the catalyst was characterized ex situ by TEM, XRD, magnetism, N-2 adsorption, TGA-DTA, and the liquid hydrolysis products were analyzed by Wet-STEM and B-11-NMR. The in situ preparation method made it possible to form nanoparticles (<12 nm) of Co2B which are the active phase for the hydrolysis reaction. In semi-batch conditions, the Co2B catalyst formed in situ is subsequently reduced by each borohydride addition and oxidized at the end of the hydrolysis reaction by OH- in the presence of metaborate. A coating of the nanoparticles has been observed by calorimetry and physico-chemical characterization, corresponding to the formation of a 2-3 nm layer of cobalt oxide or hydroxide species. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.