The microstructure of rapidly solidified copper-yttrium alloys was studied by TEM. For the near-eutectic alloys, the following observations were made : melt-spun Cu-1 2.5 wt% Y whiskers showed a very fine eutectic of f.c.c. copper and hexagonal Cu5Y plus isolated areas of a new metastable orthorhombic phase, Cu9Y; melt-spun Cu-51.1 wt% Y whiskers contained an amorphous matrix plus precipitates of the equilibrium phase orthorhombic Cu2Y. For far-from-eutectic alloys, glass formation was detected in the melt-spun whiskers of Cu-25.0 wt% Y alloy, while the melt-spun Cu-18.9 wt% Y whiskers showed a fine-grained single phase CuY5 an easily formed metastable phase, and melt-spun Cu-41.6 wt% Y showed an ultrafine single phase Cu2Y, an equilibrium orthorhombic phase.