Amorphous and well dispersed Co nanoparticles (less than 10 nm) have been in Situ synthesized in aqueous solution at room temperature. The as-synthesized Co nanoparticles possess high catalytic activity (1116 L mol(-1) min(-1)) and excellent recycling property for the hydrogen generation from aqueous solution of ammonia borane under ambient atmosphere at room temperature. The present low-cost catalyst, high hydrogen generation rate and mild reaction conditions (at room temperature in aqueous solution) represent a promising step toward the development of ammonia borane as a viable on-board hydrogen-storage and supply material. (C) 2009 Elsevier B.V. All rights reserved.