Rapidly solidified hypereutectic AlCo alloys cast by melt spinning may contain coarse intermetallic Al 9Co 2, fine Al 9Co 2 or primary aluminum dendrites with cobalt segregated interdendritically. A metallographic investigation of as-cast ribbons indicated that the volume fraction and size of the intermetallics are determined by the cooling rate and the solute content; decreasing the cooling rate and increasing the solute content tended to increase the fraction of Al 9Co 2 precipitates. The intermetallic Al 9Co 3 does not appear to result from a breakdown of primary aluminum solidification as has been previously proposed but rather from the direct nucleation of Al 9Co 2 from the melt followed by subsequent growth.