Positron annihilation spectroscopy in two versions (lifetime and coincidence Doppler broadening) has been applied to investigate solute/vacancy interactions when minor amounts (< 1wt.%) of Ag or Cu are added to the alloy Al-4Zn-3Mg (wt.%) during ageing at 150 degrees C. The results show early clustering of vacancies with Zn (and with Cu, if present). Ag displays a strong interaction with vacancies in competition with Mg and forms clusters that may help further aggregation of the other alloying elements during artificial ageing. High Mg concentration is observed at the misfit interfaces of semi-coherent or incoherent precipitates.