For the equilibrium immiscible Cu–Mo system characterized by a positive heat of formation (+19 kJ mol-1), thermodynamic calculation showed that by adding an adequate interfacial free energy, the Gibbs free energy of the Cu–Mo multilayered films could be elevated up to a higher level than the convex-shaped free energy curve of the amorphous phase and supersaturated solid solutions. Accordingly, ion beam mixing of the Cu–Mo multilayered samples did result in forming a unique amorphous phase with an alloy composition around 80 at. % Mo and two supersaturated solid solutions of fcc and bcc structures at around 27 at. % and 90 at. % Mo, respectively, matching well with the thermodynamic calculation.