The decomposition sequence of the supersaturated solid solution leading to the formation of the equilibrium S (Al2CuMg) phase in Al–Cu–Mg alloys has long been the subject of ambiguity and debate. This paper describes the identification of two distinct S phases with different lattice parameters, in a series of Al–2.5Cu–1.5Mg alloy samples, isothermally aged at temperatures between 200 and 400 °C for various times, using high-resolution synchrotron X-ray powder diffraction. The two S phases, denoted S1 and S2, form in competition with one another, with S1 being metastable with respect to S2. The lattice parameters of S2 are found to be in good agreement with those refined by Heying et al. (Z Naturforsch 2005;60B:491) for the equilibrium S phase. Despite the differences in their lattice parameters, both S1 and S2 appear to have the crystal structure proposed by Perlitz and Westgren (Ark Kemi Min Geol 1943;16B:1). The lattice parameters of both S1 and S2 are well defined, with little scatter between different samples treated for different times and temperatures, and there are no signs of the large range of lattice parameters reported by recent electron microscopy observations.