Friction stir welding of austenitic stainless steel AISI 321 to an Al-Cu alloy AA 2219-T87 (3 mm thick sheets) was investigated for a specific aerospace application. Welding experiments were carried out using WC-Co tools with different pin profiles. The effects of process parameters, including tool positioning were studied. After careful process optimization, welds with a tensile strength of close to 250 MPa were successfully produced. The welds showed a very rugged stainless steel/aluminum interface as well as some fragments of stainless steel in the stir zone, confirming complete removal of the oxide til m on the stainless steel faying surface due to the wearing action of the stirring aluminum. A very thin intermetallic layer (consisting of FeAl, Fe3Al and AlCrFe2 phases) was observed in the SS/A1 interface.