Aluminum 7XXX series alloys have been extensively used for structural components in military as well as civilian aircraft. However, stress corrosion cracking (SCC) in salt spray environments continues to be a problem for 7XXX series alloys. Transmission electron microscopy (TEM) was employed to investigate the microchemistry and microstructure of grain boundary precipitates in Al-7079 and 7075 aged at peak and over aged conditions. The grain boundary precipitates in Al 7075 at peak and over aged conditions are hexagonal η phase with stoichiometry Mg(CuxZn1-x)2. In Al-7079, however, we have observed considerable amount of T phase, (Al,Zn)48Mg32, at grain boundaries. The change in the nature of grain boundary precipitate in two different alloys affects the stress corrosion behavior. A comparison of the SCC behavior and the grain boundary microstructure will be made for Al-7079 and 7075 in this presentation.