The overall objective of this study is to investigate the microstructural stability and mechanical properties of AI-Cu-Mg-Ag alloys subjected to elevated temperatures. The addition; of Ag to Al-Cu-Mg alloys with correct Cu-to-Mg ratio has been shown to generate a precipitate phase, designated Omega, which displays superior thermal stability compared with the normally occurring S’ and Theta’. Samples produced for this study contained the expected Omega, Theta’ and S’. In addition a cubic phase, previously designated sigma(Al5Cu6Mg2), was obtained. The sigma phase was seen to be a semicoherent and coplanar phase with the At matrix, i.e., {100}//{100}(Al) and <010>(sigma)//<010>(Al). The coarsening rate of the sigma phase was found to be much lower than the Theta’ phase at 200 degrees C. An ingot was produced of the sigma phase, which was verified by X-ray diffraction. The ultrasonic technique was used to determine Young’s modulus and the shear modulus. Estimates for the structural interfacial energy were determined. A hot-stage Vickers hardness measurement on the equilibrium a phase indicates a high yield strength up to 350 degrees C. The data from the present study indicate that an Al alloy with the sigma phase may exhibit superior elevated-temperature stability.