The first measurements of Si/SiGeC valence band offsets have been performed using heterostructure p-type MOS capacitors. Single crystalline epitaxial layers of SiGeC and Si were grown by the rapid thermal CVD technique, limited reaction processing, using ethylene as the carbon source. Films with carbon contents of up to ∼1.2% were studied. X-ray diffraction analysis shows the strain compensating effect of carbon in SiGeC. Substitutionality of carbon in SiGe was also confirmed by Fourier transform infrared spectroscopy measurements. MOS capacitors were fabricated by thermal oxidation (750°C) of the epitaxial Si cap on top of the SiGeC layer. Quasi-static and high frequency capacitance-voltage measurements show wellbehaved characteristics for the samples with carbon contents up to approximately 1%. Onedimensional Poisson simulations of the C-V curves were used to extract the valence band offset between Si and SiGeC. For a fixed germanium fraction, the offset decreases with increasing carbon content. However, for a given lattice mismatch, the offset for Si/Si1−x−yGexCy is larger than the measured value for Si/Si1−zGez.