In Fe - Mo multilayered films, when the number of metal layers is kept constant, the amorphization behaviour relates closely to the thickness of the films upon energetic xenon ion mixing. In 60 nm thick films, amorphization was achieved in a narrow range around the equiatomic stoichiometry, i.e. 40 - 60 at.% Fe, while in 40 nm thick films it could be reached within a broad range of 23 - 77 at.% Fe. The grain boundary energy involved in Fe - Mo multilayered films was estimated by a simple model, and is believed to be a major thermodynamic driving force for amorphization.