The high-strength weldable 7xxx series of aluminium alloys are of great importance to the aeronautics industry. Only recently, the complex structures of the AlZnMg hardening precipitates have been solved by HAADFSTEM imaging and first-principles calculations. However, perfect models of precipitate structures are often insufficient as several elements may be mixed into precipitate compositions. We have investigated this effect by STEMEELS spectrum imaging with an aberration-corrected microscope. In a 7449 alloy, Cu and Al were found to replace atoms at certain sites in both metastable and equilibrium ZnMg precipitates.