A part of the Al–Cu–Co phase diagram adjacent to the Al13Co4 composition was studied at 800 °C by scanning electron microscopy and x-ray diffractometry. The homogeneity region of the Al13Co4 phase was found to extend up to 6 at. % Cu. At 800 °C this phase is in equilibrium with Al(Co, Cu), Al5Co2, Al9Co2, the decagonal phase, and a liquid phase. The existence of the Y-phase and Al3Co was not confirmed at this temperature.