Interfacial chemical reactions between thermally aged films of aluminum on GaAs have been observed using surface reflection Raman scattering. The measurements provide identification of the chemical composition of the reacted interfacial layer. Under thermal aging conditions in which the aluminum overlayer is not totally reacted, AlAs was observed as the reaction product. Depletion of the aluminum overlayer followed by continued reaction leads to the observation of an interdiffused AlxGa1−xAs layer.