Large undercooling can induce not only various solidification pathways, but also a precursor reaction, or liquid separation. This paper deals with the latter effect of undercooling using examples of the Cu-Co system which has a flattened liquidus. Bulk Cu-Co alloys (about 7 mm in diameter) at compositions ranging from 10 to 100 wt % Co were highly undercooled using a fluxing technique. Except for compositions above 90 wt % Co, liquid separation was directly observed as undercooling exceeded a critical value depending on the composition. It was also confirmed by a microstructural transition from dendrites to droplets above the critical undercooling. Finally, calculations of the metastable miscibility boundary were made to analyze the experimental results.