Fe-B ultrafine amorphous alloy particles have been prepared by the addition of FeSO4 solutions into KBH4 solutions. The reaction was found to automatically maintain constant pH levels in a basic medium and produced Fe-B samples with higher boron content as compared to the reaction for the addition of KBH4 solutions into FeSO4 solutions. The latter reaction automatically maintained constant pH levels in an acidic medium. The mechanism of the overall reaction between FeSO4 and KBH4 in aqueous solution is composed of three simple independent reactions which are shown to be valid for both addition routes for the reaction. The concentration and addition rate of FeSO4 solutions were also found to significantly influence the boron content and properties of the produced Fe-B samples.