Prior studies on the grain boundary segregation of Al-Zn-Mg alloys have shown that Mg and Zn are heavily segregated to the grain boundaries. AES studies coupled with plasmon-loss measurements have enabled us to identify the chemical state of the segregants present on the grain boundary. These studies show that in the asquenched condition, the Mg and Zn segregated along the grain boundaries are present in solid solution. In the peak-and over-aged conditions, all the Zn is incorporated into the MgZn2 particles while a major fraction of Mg is present as free Mg. In the as-quenched condition, the segregants are confined to about 50 to 100 angstrom from the grain boundary. Utilizing the vacancy solute drag model, it is possible to qualitatively account for the observed extent of solute segregation. The implications to stress corrosion cracking SCC of the free Mg observed on the grain boundaries under all heat treatment conditions can be analyzed by proposing Mg-H complex formation. Consequently this years efforts were focused on studying the effect of the MgZn ratio on the SCC Susceptibility of Al-Zn-Mg alloys.