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Evolution of plate-like precipitates of the Ω phase in an Al–1.9Cu–0.3Mg–0.2Ag (at.%) alloy has been investigated by three-dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques. In an as-quenched alloy, all solute atoms are uniformly dissolved. After 5s aging at 180°C, co-clusters of Ag and Mg atoms are present. Initially, these co-clusters do not have well-defined shape, but after aging for 120s, they are aligned on the {111} planes incorporating Cu atoms. Initially, they do not have the distinct structure consistent with the Ω phase, thus these are designated as {111}GP zones. The Ω platelet which appears after the {111}GP zones contain all solute elements within the precipitate having a composition of approximately 25at.% Cu, 20at.% Mg and 10at.% Ag. In the well-defined Ω phase which appears after 2 and 10h aging at 180°C, Ag and Mg atoms are strongly segregated to the …