Room temperature 200 keV xenon-ion irradiation of multilayers, designed under interfacial free energy concern, did result in the formation of Mo–Cu metallic glasses only within a narrow Mo-enriched composition range. In addition, some Cu-based alloys of metastable fcc solid solutions were also formed at different irradiation stages. To obtain a possible thermodynamic interpretation, a Gibbs-free-energy diagram was calculated based on the semiquantitative Miedema’s model and the related methods proposed by Alonso. The free energy of the Mo–Cu multilayers including excess interfacial free energy was also estimated and was compared to that of the amorphous state. It turned out that the interfacial free energy stored in the multilayered films could raise the energy level of the multilayers to a state higher than that of the metastable phases and thus played an important part, in comparison to the energy deposited by irradiation, in inducing alloying in this immiscible system. ©1997 American Institute of Physics.