Welding of aluminium alloy, belonging to the 6XXX series, exerts an adverse influence on its strength. The loss of strength immediately adjacent to the welds can have an influence on overall behaviour of the structure or component. In this study, the technique of gas metal arc welding was used. The filler material used was a silicon-containing aluminium alloy. Subsequent to welding, the alloy (6061-T4) was subjected to post-weld heat treatment (PWHT) at 185°C for 6 h. Both the as-welded and artificially aged extrusions of aluminium alloy 6061 were examined for both microstructural development and resultant influence on mechanical properties. Light optical microscopy was used to characterise the microstructure of the starting material. The PWHT had a marginal influence on both the presence and distribution of second-phase particles in the microstructure. The temperature and time of heat treatment should result in the precipitation and presence of very fine precipitate particles in the microstructure. Tensile tests revealed the welded aluminium alloy to have lower strength, both yield strength and ultimate tensile strength, when compared to the as-provided un-welded counterpart. The influence of PWHT on strength, ductility and fracture properties of the alloy, in comparison with the as-provided material, is presented and discussed.