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The Al-Au-Cu system and its associated ternary alloys and intermetallic compounds is surprisingly poorly known, and the authors could find no phase diagram for it in the literature. This paper addresses this omission by presenting an isothermal section at 500°C, derived with the aid of X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), metallography and hardness measurements. The samples studied had generally received an anneal of 2 hours at 500°C, primarily in order to complete any transformations that occurred during solidification and cooling of the castings. The possibility of further changes on protracted annealing at 500°C is not ruled out, and the diagram presented is therefore applicable only to material prepared by thermal processing of an industrial nature. The presence of a ternary β phase with a nominal stoichiometry of AlAu2-xCu1+x, 0≤x≤1 was confirmed, and its phase field at 500°C determined. A number of the binary intermetallic phases were found to exhibit some solid solubility of the ternary element. In particular, the γ Al4Cu9 phase extends deep into the ternary and, in the vicinity of the commercially interesting 18 carat line, appears to exist in a ternary ordered form, designated here as γ2