Surface reflection Raman scattering has been used to examine interfacial chemical reactions which occur at the Al–GaSb interface during thermal aging. The chemical composition of the interfacial layer has been determined and reactions have been identified at temperatures as low as 300 °C. Under thermal aging conditions where the Al overlayer is not fully reacted, the primary reaction product is AlSb. Depletion of the aluminum overlayer followed by continued reaction leads to an interdiffused layer of AlxGa1−xSb which exhibits a concentration gradient normal to the interface. Both amorphous and crystalline AlSb have been identified depending on the thermal aging conditions.