Abstract: The phase equilibria and compositions of three solid solutions in the low Cu region of a Al-Zn-Cu ternary system were investigated by means of diffusion couple-EPMA technique. It was found that Zn contents in α1 and α2 both decrease due to Cu additions when α1/α2 and α2/β are both in equilibrium at 320°C; Zn content in α increases with the small amount of Cu when α/β is in equilibrium at 260°C. The distribution ratios of Cu are all unsymmetrical when α1/α2, α2/β and α/β are all in equilibrium, i.e., Cu content in the latter phase is much more than that in the former one. This means that the α1/α1+α2 phase boundary approaches to Al-rich corner due to Cu additions at 320°C, and the α/α+β phase boundary is along the increasing direction of Zn content due to Cu additions at 260°C.