Specimens of Al-Zn-Mg alloys containing 2 to 3 at. % Zn and 0. 11 to 3. 90 at. % Mg were used to determine the (mean) transport energies of the diffusing complexes during aging at 20 to 40 C via typical values in the time dependence of the modulus of elasticity. The results indicate that below a critical Mg/Zn ratio of 1/3, the behavior of the alloys resembles that of zinc but above that ratio it resembles that of magnesium. A mechanism is suggested.