Grain‐boundary surfaces of Al‐Zn‐Mg alloys have been studied with Auger electron spectroscopy. Particular emphasis was given to plasmon loss satellites in the Auger spectrum. It is found that the plasmon energy losses associated with Al, Zn, and Mg Auger peaks are different, and the energy losses vary with heat treatment of the alloy. These variations provide information on chemical composition and nonhomogeneities in an alloy, such as precipitation and segregation. It is shown that, at the grain boundary of aged specimens of the Al alloy, the Zn atoms are all in the form of MgZn2 precipitates, but only about 40% of the total Mg is in the precipitates. The other 60% is situated outside of the precipitates.