This work explores how the Cu/Mg ratio affects the thermal stability of the extrusion of Al-4.6 pct Cu-Mg-Ag alloys at aerodynamic heating temperatures of up to 155°C for 1000 hours. The Cu/ Mg ratio was modified by adding various amounts of elemental Mg. The alloy microstructures, which determine thermal stability, were analyzed with an optical microscope, an electrical conductivity meter, a differential scanning calorimeter (DSC), and a transmission electron microscope. Experimental results indicate that reducing the Cu/Mg ratio of the alloy by the further addition of Mg causes X to become the primary strengthening phase after aging treatment, increasing the hardness and the thermal stability, although the h¢ phase is suppressed. The S phase does not affect the thermal stability of the alloy, because it is present in only a low quantity. Results of this study also demonstrate that the hot-extruded low Cu/Mg alloy satisfies the requirement of the material thermal stability of commercial supersonic aerial applications.