Sodium borohydride NaBH4-reduced cobalt is a very attractive material in catalyzing NaBH4 hydrolysis, as it is highly reactive and cost-effective. However, the nature of its catalytically active phase is still unknown although it has been mostly suggested as being either cobalt boride such as Co2B or an alloy Co-B. With the objective to contribute to highlight the real nature of the catalytically active phase, we performed a systematic study focusing on synthesizing different Co catalysts, characterizing them by ICP-AES, N-2 adsorption/desorption, EDS, FTIR, DSC, SEM, TEM and XRD, and testing them in NaBH4 hydrolysis. Herein, it is showed that, to be highly reactive, the Co catalyst has to be reduced by a boron-containing reducing agent such as NaBH4 and ammonia borane NH3BH3. Furthermore, it has to be amorphous. None of both features were observed in the case of the hydrazine N2H4-reduced cobalt. Hence, it is suggested that the presence of boron is necessary to achieve better reactivity through electronic and geometric effects. (C) 2011 Elsevier B.V. All rights reserved.