The effect of increasing the contents of Cu, Ni and other alloying elements on the microstructure and elevated temperature mechanical properties of Al-12Si eutectic alloys was investigated. Increased Cu and Ni contents resulted in improved tensile properties after T5 and over-aging at 350degreesC treatments. After T5 heat treatment, a dense precipitation of theta' precipitates was observed and was considered to be the main strengthening sources at this stage. However, during aging at 350degreesC, these precipitates were transformed into a coarse Q-phase(Al5Cu2Mg8Si6). The elevated temperature tensile properties of the test alloys after prolonged aging at 350degreesC could well be related to the increased volume fraction of the intermetallic phases.