NaBH4 and KBH4 hydrolysis reactions (BH4- + 4H(2)O - B(OH)(4)(-) + 4H(2)), which can be utilized as a source of high purity hydrogen and be easily controlled catalytically, are exothermic processes. Precise determination of the evolved heat is of outmost importance for the design of the reactor for hydrogen generation. In this work we present an efficient calorimetric method for the direct measurement of the heats evolved during the catalyzed hydrolysis reaction. A modified Setaram Titrys microcalorimeter was used to determine the heat of hydrolysis in a system where water is added to pure solid NaBH4 or KBH4 as well as to solid NaBH4 or KBH4 mixed with a Co-based solid catalyst. The measured heats of NaBH4 hydrolysis reaction were: -236 kJ mol(-1), -243 kJ mol(-1), -235 kJ mol(-1), and -236 kJ mol(-1), without catalyst and in the presence of Co nanoparticles, CoO and Co3O4, respectively. In the case of the KBH4 hydrolysis reaction, the measured heats were: -220 kJ mol(-1), -219 kJ mol(-1), -230kJ mol(-1), and -228 kJ mol(-1), without catalyst and with Co nanoparticles, COO and Co3O4, respectively. Also, a comparison was made with an aqueous solution Of CoCl2 center dot 6H(2)O used as catalyst in which case the measured heats were -222 kJ mol(-1) and - 196 kJ mol(-1) for NaBH4 and KBH4 hydrolysis, respectively. The influence of solid NaOH or KOH additions on the heat of borohydride hydrolysis has been investigated as well. (C) 2009 Elsevier B.V. All rights reserved.