Co-B catalysts were prepared by the chemical reduction of COCl(2) with NaBH(4) for hydrogen generation from borohydride hydrolysis. The catalytic properties of the Co-B catalysts were found to be sensitive to the preparation conditions including pH of the NaBH(4) Solution and mixing manner of the precursors. A Co-B catalyst with a very high catalytic activity was obtained through the formation of a colloidal Co(OH)(2) intermediate. The ultra-fine particle size of 10 nm accounted for its super activity for hydrogen generation with a maximum rate of 26 L min(-1) g(-1) at 30 degrees C. The catalyst also changed the hydrolysis kinetics from zero-order to first-order. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.