The effect of Ag addition in high strength Al-4wt%Cu-1.5wt%Si-1.0wt%Mg has been studied by optical microscope, x-ray diffractometer, energy dispersive spectrometer, transmission electron microscope and differential scanning calorimeter. Special emphasis was placed on investigating the decomposition and formation kinetics of Al₂Cu phase in rapidly solidified and squeeze cast alloys. The microstructure, in general, consists of primary solidification phase of α-Al and secondary solidification phases of Al₂Cu, Al₂CuMg and Mg₂Si. Detailed microstructural and thermal analyses showed that addition of Ag accelerated the decomposition behaviour of the Al₂Cu secondary solidification phase. The number of catalytic nucleation sites in rapidly solidified Al-4wt%Cu-1.5wt%Mg and Al-4wt%Cu-1.5wt%Mg-0.7wt%Ag alloys were physically unreal, i.e., 8.0×10^(-13) and 5.1×10^(-14), respectively. However, The sharp exothermic peak of Al-Cu-Mg-Ag, different from Al-Cu-Mg, shows that Ag is good dopant for heterogeneous nucleation. This result demonstrates the important of chemical factor in determining the catalytic efficiency of heterogeneous nucleants. (Received July 20 1998)