EDX spectroscopy is carried out to trace chemical composition profiles across the grain boundary in three different tempers : T651(peak aged), T7951(slightly over-aged) and T7651(over-aged) and for three plate thicknesses : 10 mm, 15 mm and 25 mm. Quantitative assessment of the concentration profiles indicates : (1) The concentration variation of Zn across the grain boundary is much more marked than that of Mg or Cu, irrespective of the aging and quenching conditions. (2) The minimal concentration of Zn occurring in the matrix decreases significantly on passing from T651 to T7951, as is the case for the minima observed in the PFZ. (3) The variation of the minimal concentration of Cu and Mg is less marked than that of Zn. Nevertheless, a trend of lowering with over-ageing is observed for the minima occurring in the PFZ, as well as in the matrix.