The clustering kinetics in quenched pure or Sn-containing Al-Mg-Si alloys during natural ageing (NA) was investigated by positron annihilation lifetime spectroscopy.From the perspective of vacancies, the experimental results show that the rate of solute clustering during NA can be notably reduced by adding only 40 or 70 ppm of Sn.Three plausible reasons account for such a retardation: (i) the total concentration of quenched-in vacancies (V) available for the diffusion of Mg and Si atoms becomes considerably lower due to the immobilisation of vacancies by Sn atoms, i.e.much stronger binding energy between V-Sn than between V-Mg and V-Si;(ii) the migration of Mg and Si atoms assisted by the remaining vacancies might be affected by Sn atoms;(iii) the diffusivity of V-Sn complexes at "room temperature" could be extremely slow, originating from the much larger atomic volume of Sn compared to Al.All these effects together lead to the observed sluggish clustering kinetics in Sn-containing alloys.The key to suppress the detrimental effect of NA on subsequent artificial ageing (AA) in industrial Al-Mg-Si alloys lies in controlling vacancy-assisted diffusion of solute atoms.