Fusion welding of dissimilar aluminum alloys is very challenging. In the present work, Al-Cu alloy AA2219-T87 was friction stir welded to Al-Mg alloy AA5083-H321. Weld microstructures, hardness, and tensile properties were evaluated in as-welded condition. Microstructural studies revealed that the nugget region was primarily composed of alloy 2219, which was placed on the advancing side. No significant mixing of the two base materials in the nugget region was observed. Hardness studies revealed that the lowest hardness in the weldment occurred in the heat-affected zone on alloy 5083 side, where tensile failure were observed to take place. Tensile tests indicated a joint efficiency of around 90%, which is substantially higher than what can be achieved with conventional fusion welding. Overall, the results show that satisfactory butt welds can be produced between AA2219-T87 and Al-Mg alloy AA5083-H321 sheets using friction stir welding. © 2010 TIIM, India.