The Co-Mo system has a small negative heat of formation (ΔH f = -7 kJ/mol) and its composition favoring amorphization is limited in a restricted region. In a multilayer scheme, an extra interfacial free energy stored in the films can extend the composition range for forming Co-Mo amorphous alloys. Multilayered films with totally 30 metal layers were designed to have various overall compositions and prepared by alternate-deposition at varying rates and switching times between two depositions. It turned out that amorphous Co-Mo alloy films were indeed formed directly after deposition under appropriate conditions and their composition was in a relatively broadening region from 40 to 82 at% of Co. The calculated Gibbs free energy diagram gives a reasonable interpretation of the observed amorphization as well as the compositional dependence of structural evolution of the formed amorphous Co-Mo alloys upon post-annealing. Besides, the changes of magnetic properties upon post-annealing of the deposited films may also serve as an indirect evidence of the observed structural evolution. The formation of amorphous Co-Mo alloys is attributed to an in-situ interfacial reaction driven by the interfacial free energy together with a utilization of the heats of condensation during vapor-to-solid transition.