Abstract The present paper reports TEM investigation on the detailed microstructure and its formation of rapidly solidified CuCo alloys. Results show that the previously observed f.c.c. α cobalt phases in some of the rapidly solidified CuCo alloys are precipitates formed when the ribbon loses its intimate contact with the substrate wheel. The solid solubility of cobalt in copper could further be extended beyond the previously reported value by taking appropriate measures during melt spinning. The f.c.c. precipitates with twins are parallel and coherent with the matrix.