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 film 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/Al interface. © 2016 by The Minerals, Metals & Materials Society.