An estimation for the band offsets and the fundamental band gap will be presented for Si1-x-yGexCy alloys tensile or compressive strained on Si(001). This estimation considers both the band lineup at the interface of two different materials as well as the strain effects. Unknown material parameters have been adjusted to obtain the best agreement with experimental results for tensile strained Si1-yCy layers. The obtained results agree very well with the first experimental data for the effect of C on band-structure properties in Si1-x-yGexCy. For a completely strain-compensated (cubic) Si1-x-yGexCy layer, we predict significant “Ge effects” (smaller gap than Si, valence-band offset to Si) with values depending on the Ge content. © 1998 American Institute of Physics.