Cobalt boride (Co2B) is an inexpensive catalyst for the hydrolysis of sodium borohydride (NaBH4) to generate hydrogen (H-2) for PEM fuel cells. Preparation Of Co2B in situ in the H2 generation vessel as well as in an external reactor was studied using cobalt (III) oxide (Co3O4) and lithium cobalt oxide (LiCoO2) as precursors. The oxide precursors were characterized by XRD and the Co2B catalysts by XRD and ICP. Co2B formation from the oxide precursors depends on the crystallinity of the oxides as well as the concentration of NaBH4 in the solution. During reduction of the oxides to CoB2, Co(s) metal, Co(BO2)(2) may also form from competing side reactions, however crystalline oxides react faster leading to higher Co2B yield and better H-2 generation efficiency. Crystallinity Of Co3O4 was improved by preparing it at a higher temperature. Co3O4 prepared at 600 degrees C reacts faster leading to enhanced Co2B formation than Co3O4 prepared at lower temperatures (200 degrees C and 400 degrees C). The activation energy for the hydrolysis of NaBH4 by Co2B formed in situ from Co3O4 produced at 600 degrees C was calculated to be 77.90 kJ mol(-1). This activation energy value is found to be slightly higher than that of Pt, Ru-based catalysts. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.