Amorphous films of 300 nm thick were obtained by solid state reaction in an immiscible Y–Mo system with a positive heat of mixing being +26 kJ/mol. The experimental results revealed that the Y51Mo49 multilayers, including 19% interfacial atoms, were vitrified by thermal annealing at 300 °C for one and half hours, and that the formed amorphous films recrystallized at about 600 °C. These confirmed that the excess interfacial free energy stored in the multilayered films elevated the initial energetic state of the multilayers to a level higher than that of the amorphous phase, providing a major driving force in solid state amorphization. The elastic modulus and hardness changes of the formed amorphous films upon annealing were characterized by nanoindentation, and the results were in accordance with the proposed free volume model for the amorphous structure. ] © 1996 American Institute of Physics.