The cobalt-catalyzed hydrolysis of sodium borohydride (NaBH(4)) has become an attractive process in view of the possibilities of using the hydride for hydrogen storage material and also for the production of amorphous and tunable-size magnetic nanoparticles. This process in which the metallic catalyst transforms into a Co- and B-based magnetic by-product when in contact with NaBH(4) has been modified in order to control the mechanism of formation, tune the final size and study the particular magnetic behavior of the Co-B alloy nanoparticles provided.