The direct and indirect band gaps of Si1−x−yGexSnyare inferred from the calculated energy‐band structure of &agr;‐Sn and from the known structures of Ge and Si. Our assumptions are: that the energy‐band shapes of the binaries Sn1−xGex, Ge1−ySiyand Si1−ySnychange smoothly withxandy, and that the energy gap of SiGeSn can be estimated by interpolation from the gaps of SnGe, GeSi, and SiSn. The optical indices of refraction of SiGeSn are also estimated.