Recent results on metastable semiconducting alloys, concerning in particular the growth of new Sn-based alloys (GaSb)1−x(Sn2)x and Gel−xSnx and the physical properties of (GaAs)1−x(Ge2)x and (GaSb)1−x(Ge2)x, are discussed. (GaSb)1−x(Sn2)x and Ge1−xSnx alloy films were grown with x-values as high as 0.20 and 0.15, respectively, well in excess of equilibrium Sn solid solubility limits (<1%) while epitaxial (GaAs)1−x(Ge2) and (GaSb)1−x(Ge2)x alloys were obtained on (100) GaAs at compositions ranging across the pseudobinary phase diagram. Low energy ion bombardment induced collisional mixing and preferential sputtering during film growth played a critical role in obtaining single phase alloys. An optimal ion energy, which depended on the ion flux and the alloy composition, was determined, allowing in most cases growth at temperatures T, sufficient for obtaining single crystal alloys on (100) GaAs and (100) Ge substrates. Decomposition of the Sn-based alloys occurred above a critical Ts- value via α-Sn-rich precipitates which were stable above the β-Sn melting point. X-ray diffraction, STEM, EXAFS, and Raman spectroscopy measurements, performed on single crystal (GaAs)1−x(Ge2)x and (GaSb)1−x(Ge2)x alloys, indicate that there is a transition in the long-range order from zincblende to diamond with increasing x while the short-range order remains perfect at all compositions, i.e. no V-V or III-Ill bonds are observed. These results are discussed in light of recent models which relate (GaAs)1−x(Ge2)x atomic structure to its band structure and optical properties.