Friction stir welding (FSW) is a relatively new welding technique, which makes it possible to weld low fusion weldability materials with very little or no post weld distortion, high mechanical strength and good fatigue performance. This is particularly important for welding high strength precipitation strengthened aluminium alloys of the 7xxx series previously not recommended for welding. Residual stress measurements have been made, using the synchrotron X-ray diffraction transmission technique, on a 7xxx series aluminium alloy plate made by friction stir butt welding two extruded T-profiles along their middle section. Measurements were also carried out on a non-welded extruded section to determine the pre-welded strain state. Comparisons have been made with holographic interferometry method. Hot extrusion of thin walled sections of 7xxx series alloys results in substantial preferred orientation. The grains appear mainly equiaxed in the transverse and highly elongated in the longitudinal directions, which is described as fibre texture. During friction stir welding, the original extrusion texture is destroyed. In the stirred region, fine grained recovered microstructure is observed, whereas in the thermo-mechanically affected zones, it is intermediate. This strongly textured microstructure makes is difficult to accurately determine the residual stress fields using the traditional measurement techniques but non-destructive synchrotron strain scanning produced excellent results over a full cross-section of the weld.