The microstructure of a friction stir-welded Al-6.0Cu-0.75Mg-0.65Ag (wt.%) alloy in the peak-aged T6 temper was characterized by transmission electron microscopy. Strengthening precipitates found in the base alloys dissolved in the weld nugget, while those in the heat-affected zone coarsened considerably, causing softening inside the weld region. Precipitates of the Omega (Al2Cu) phase, being the main strengthening phase in base material, grew up to 200-300 nm in the heat-affected zone, but their density decreased. They co-existed with Theta' (Al2Cu), S' (Al2CuMg), Theta (Al2Cu) and sigma (Al5Cu6Mg2) phases. The density of the Theta' and S' phases as well as their sizes increased compared to the base material. The high-resolution observation allowed to compare the morphology of the Q phase plates in the heat-affected zone and in the base material. (C) 2003 Elsevier Science B.V. All rights reserved.