Optical absorption measurements have been performed to study the effect of substitutional carbon on the valence band offset of compressively strained p+ Si1-x-yGexCy/(100) p- Si. The compressively strained p+ Si1-x-yGexCy/(100) p- Si heterojunction internal photoemission structures were grown by rapid thermal chemical vapor deposition with substitutional carbon levels up to 2.5%. Carbon decreased the valence band offset by 26±1 meV/% substitutional carbon. Based on previous reports of the effect of carbon on the band gap of Si1-x-yGexCy, our work suggests that the effect of carbon incorporation on the band alignment of compressively strained Si1-x-yGexCy/Si is to reduce the valence band offset, with a negligible effect on the conduction band alignment. © 1998 American Institute of Physics.