One main limitation in the strength of welds in Al-Zn-Mg alloys consists in the low temperature stability of the precipitates initially present in the material, which involves phenomena such as dissolution, coarsening and transformation from metastable particles to more stable ones. The aim of our study is to determine quantitatively the modification of the microstructural features involved by arc-welding (MIG) of a commercial Al-Zn-Mg alloy, especially in the heat-affected zone. First, the precipitation state just after welding has been measured in each point of the heat-affected zone using Small-Angle X-ray Scattering (SAXS). Two initial states have been compared : T4 (containing mainly GP zones) and T7 (containing mainly stable eta-precipitates). The capacity of the material to recover a high and uniform strength through post-weld heat-treatments has also been investigated using microhardness and SAXS. Both natural ageing (at room temperature) and two-step ageing treatment (at 170degreesC) have been performed on the two initial welded structures.