Heat treatable Al-Zn-Mg alloys can be strengthened by the precipitation of second phase particles. In this paper, Al-6.57%Zn-2.83%Mg and Al-6.57%Zn-2.83%Mg-3.92%Cu alloys (in wt%) in T7 state (140 A degrees C for 96 h) have been prepared. The effects of Cu and Al on the concentration and structure of equilibrium eta (MgZn2) phase have been investigated by high resolution transmission electron microscopy, aberration-corrected scanning transmission electron microscopy, selected election diffraction pattern simulations, and first-principles calculations. The effects of Cu and Al substitution on the diffraction characteristics of the eta phase and the general rule of Cu and Al substitution in the eta phase have been discussed.