Phase formation has been studied in Au‐Al thin‐film interactions by means of4He+MeV backscattering and x‐ray diffraction techniques. The films were annealded at temperatures low enough for no liquid phase formation. Au4Al, Au5Al2, Au2Al, AuAl, and AuAl2were detected from samples prepared by depositing the exact stoichiometry quantities of Au and Al. The phase formation sequence was determined. The first phase formed is Au5Al2; the others are observed with the disappearence of one phase previously formed. For instance, Au4Al grows only when all Al is reacted and, on the other hand, AuAl2when all Au is reacted. The sequence of the phases in the Au‐Al interactions cannot be interpreted on the basis of pure thermodynamic or kinetic arguments and a composite mechanism is proposed.