We studied ion beam mixing of thin Mo films on monocrystalline Si by As “implantation at room temperature. The results differ significantly from those obtained for implantation at elevated temperature (T > 200°C). where ion beam mixing results in hexagonal MoSi2 formation. Room temperature implantation results in the formation of an amorphous mixed layer. The composition of this layer varies with depth from Mo-rich to Si-rich. The mixed layer thickness increases linearly with implanted dose and energy. An increase of the implantation temperature with 100°C gives rise to a factor of 2 larger mixed layer thickness and to the formation of amorphous MoSi2 near the interface with Si. These phenomena indicate that at elevated temperature ion beam mixing is controlled by radiation-enhanced diffusion whereas, at room temperature ballistic mixing is the dominant mechanism.