As to Al-Zn alloy, the difference between the formation of
G.P.zones and that of clusters was investigated by measurements of electrical resistivity. The results obtained were summarised as follows: (1) G.P.zones formed during the quench and quenched-in vacancies increase greatly as-quenched resistivity P(o) as quenching temperature Tq is raised, and clusters increase slightly P(o) as Tq is lowered. (2) For one Tq and one Ta, the time required to reach P(e)' for Al-1.3at % Zn alloy is longer than that for Al-3.0at % Zn alloy. This is due to the difference of number of zinc atom in the clusters. (3) For one Ta and one concentration of zinc, the time required to reach p'(e) at Tq = 170℃ is longer than that at Tq = 300℃. This is due to the difference in concentration of quenchedin vacancies. As to Al-Cu alloy, the solvus temperature for G.P.zones was determined from the existence of P(m) in ageing curves by measurements of electrical resistivity. Consequently the solvus temperature is between 20℃ and 60℃.