As-cast alloy ingots of nominal compositions Al65Co20Cu15, A160Co20Cu20, Al65Co15Cu20, and Al70Co15Cu15were studied with transmission electron microscopy and energy-dispersive X-ray spectroscopy. A compositional variation among the grains, in each alloy, allowed an examination of phase formation as a function of the ternary composition near the nominal compositions. Alloys of composition Al65Co20Cu15, often taken as a stable decagonal phase, instead form a large (about 100 Å) unit-cell crystal phase that approximates the decagonal phase. No decagonal phase forms near this composition. True decagonal phase formation is only observed in a narrow phase field centred near Al65Co15Cu20.