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Contactless electroreflectance is applied to study direct optical transitions from the heavy hole, light hole, and spin-orbit split-off band to the conduction band in compressively strained Ge 1− x Sn x layers of various Sn concentrations at room temperature. It is shown that the energies of these transitions are in very good agreement with theoretical predictions, which take into account non-linear variation of bandgap and spin-orbit splitting plus the strain-related shifts obtained from the Bir-Pikus theory. The bowing parameter for the direct bandgap has been determined to be 1.8±0.2 eV and agree with this one obtained within ab initio calculations, which is 1.97 eV (for indirect bandgap the bowing parameter is 0.26 eV).