The alloying behavior in amorphous antimony (a-Sb) clusters has been studied using transmission electron microscopy. Antimony was first evaporated onto an amorphous carbon film and nanometer sized a-Sb clusters were produced. Gold, copper or lead was then evaporated onto a-Sb clusters on the film kept at room temperature. When gold atoms are deposited, they quickly dissolve into a-Sb clusters to form clusters of the AuSb2 compound. In the case of copper, copper atoms dissolve into a-Sb clusters and clusters of amorphous Sb-Cu alloy are formed. In contrast, in the case of lead, no alloying of lead takes place and crystallization of amorphous antimony is induced. The present results provide evidence that the heat of alloy formation plays an important role in determining the ease with which the spontaneous alloying takes place in nanometer sized ultra-fine particles.