Cobalt clusters–silica nanospheres (15–30 nm) were synthesized using a Co(NH3)6Cl3 template method in a polyoxyethylene-nonylphenyl ether/cyclohexane reversed micelle system followed by in situ reduction in aqueous NaBH4/NH3BH3 solutions. The cobalt clusters are located either inside or on the outer surface of the silica nanospheres as shown by the transmission electron microscope (TEM)/energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) measurements. The cobalt–silica nanospheres have a high catalytic activity for the hydrolysis of ammonia borane that generates a stoichiometric amount of hydrogen, and can be efficiently cycled and reused 10 times without any significant loss of the catalytic activity.