The precipitation characteristics in Al-5wt% Zn-2wt% Mg alloys with or without a trace addition of Ag, Cu or Cr have been studied as the function of solution temperature and time, quenching temperature and medium, and pre-aging between quenching and artificial aging mainly by electrical resistivity measurement and tensile test. Imperfections such as sub-boundaries still remained after incomplete solution treatment, though it is particularly the case when added Cr, influence remarkably the following precipitation. Vacancy-solute clusters, which could be formed during or immediately after quenching, can act as precipitation nuclei of η' intermediate phase and, therefore, influence considerably the age-hardening at higher temperatures. Pre-aging and small additions of Ag have also many favorable effects on higher temperature aging
