Synthesis of strain‐compensated single‐crystal Siy(SnxC1-x)1-y alloy films on Si (100) substrates has been achieved with compositions of Sn and C greatly exceeding their normal equilibrium solubility in Si. Amorphous SiSnC alloys were deposited by molecular beam deposition from solid sources followed by thermal annealing. In situ monitoring of crystallization was done using time‐resolved reflectivity. Solid phase epitaxy for Si0.98Sn0.01C0.01 occurs at a rate about 20 times slower than that of pure Si. Compositional and structural analysis done using Rutherford backscattering, electron microprobe, ion channeling, x‐ray diffraction, and transmission electron microscopy are consistent with substitutional C and Sn, and with defect‐free single crystal films.