The impact of cooling rates on the microstructure of Cu-Co and Cu-Fe-Co alloys was investigated by scanning electron microscopy. The high cooling rates entailed in the electron beam surface melting of the alloys result in bulk supercooling of at least 150 K, which in turn causes three microstructural effects: (i) melt separation into two liquids, namely copper poor, L1, and copper rich, L2; (ii) microstructural refinement; (iii) enhanced solute trapping of Cu in the Fe- or Go-rich phases. No evidence of metastable liquid separation was found for Cu-50 wt % Co. There are indications that similar dynamic supercooling exists in copper-quenched or are-melted samples near the splat contact.