Solute and vacancy depletion have long been investigated to reveal the formation mechanism of grain boundary precipitate-free zones (GB-PFZ) during ageing, yet there is no conclusive explanation due to the simultaneous appearance of the two in GB-PFZ. In this study, the evolution of GB-PFZs and solute distributions in the vicinity of grain boundaries (GBs) were studied during the homogenisation of an Al-Cu-Mg-Mn alloy using transmission electron microscopy, high-angle annular dark field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Results indicated that the evolution of GB-PFZ during homogenisation can be divided into the following three stages: Stage I, formation and recession of GB-PFZ; Stage II, absence of GB-PFZ, and Stage III, the reappearance and broadening of GB-PFZ. The results also revealed that the GB-PFZ in Stage I is totally devoid of solute depletion and its formation can be attributed to vacancy depletion alone. The GB-PFZ at Stage III solely caused by solute depletion and excludes vacancy depletion.