Single-phase SnxGe1-x alloys with x up to 0.3 have been grown by molecular beam epitaxy. X-ray diffraction measurements indicate the layers to have the diamond crystal structure. The metastability of the alloys is apparent as increases in the growth temperature, layer thickness, or Sn composition cause phase separation of the Sn into a noncubic (white or β-Sn) form. Rutherford backscattering spectrometry and reflection high-energy electron diffraction measurements indicate that the initial stages of growth are complicated. The first several hundred angstroms of growth are compositionally graded, with the Sn incorporation rate increasing with film thickness. Thereafter, the alloy composition remains constant, determined by flux composition, until a critical thickness for phase separation is reached (≅2000 Å for x=0.3).