The group IV metastable Sn x Ge 1− x alloy system is an interesting semiconductor material with potential applications in the fabrication of Si-based heterojunctions and long wavelength infrared optoelectronic devices. Band structure calculations have suggested that the Sn x Ge 1− x alloys may have direct energy gaps continuously tunable from 0.55 to 0 eV for compositions x from 0.2 to 0.6 with very small electron effective masses and hence high electron mobilities. However, syntheses of Sn x Ge 1− x alloy films in the direct gap composition range by conventional epitaxial or polycrystalline thin film growth techniques have not been successful due to the severe surface segregation of Sn during the film growth. In this work, we report the synthesis of epitaxial Sn x Ge 1− x /Ge/Si(001) with compositions up to x =0.34 by ion-assisted molecular beam epitaxy with 30–100 eV Ar + ions produced by an electron cyclotron resonance ionization source with ion to atom flux ratios of the order of unity in the substrate temperature range of 120 to 200°C. High flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy. In situ reflection high energy electron diffraction as well as X-ray rocking curve indicated epitaxial Sn x Ge 1− x alloy films, and Rutherford backscattering spectra confirmed the Sn x Ge 1− x alloy compositions and indicated an absence of Sn segregation.