Electromagnetic levitation was used to determine Cu-Nb phase diagram and to study supercooling effects on solidification characteristics of the alloys containing 5-70 wt% Nb. The Cu-Nb stable phase diagram was found to exhibit near-flat liquidus with a peritectic reaction at 1093 A degrees C. Melt separation was found only for specimens containing approximately 20 wt% Nb. The results indicate that melt separation in the alloy requires supercooling exceeding 230 K combined with high cooling rates during solidification. Some specimens quenched from the solid + liquid zone on a copper chill also show evidence of melt separation which is attributed to minor oxygen impurities. Nb-rich liquid which nucleates below the T (0) curve solidifies as a metastable Nb-bcc lattice containing only 67 wt% Nb as compared to 96 wt% of the regular Nb dendrites.