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We have investigated the origin of the initial rapid hardening of an Al–1.3 at.% Mg–1.7 at.% Cu alloy by coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopy. Quenched-in vacancies are bound to Mg atoms rather than Cu atoms initially and the vacancy–Mg complexes easily migrate to vacancy sinks at 150°C. Vacancy–Mg–Cu complexes form during the initial 1 min aging at vacancy sinks, meanwhile vacancy density decreases rapidly. These results support that the dislocation–solute interaction is the origin of the initial rapid hardening.