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Effects of the trace element germanium (Ge) on the quench sensitivity of an Al-Zn-Mg-Zr alloy were investigated in the present work. The results showed that the Ge-bearing alloy exhibited lower quench sensitivity as compared to the Ge-free alloy. This phenomenon could be reasonably interpreted in terms of the stability of supersaturated solid solution of alloys after quenching from an elevated temperature. The apparent vacancy formation energies for the Ge-free and Ge-bearing alloys were determined to be 0.49 and 0.58 eV respectively. This suggested that the addition of a small amount of Ge was able to trap excess vacancies, leading to a decrease in the amount of coarse dispersoids and resultant low quench sensitivity in Ge-bearing alloys. Therefore, Ge could be used in alloy productions, which require a slow cooling rate to reduce the residual stresses and distortions. [ABSTRACT FROM AUTHOR]