We report on conduction and valence band offsets in thick, relaxed Ge-rich Si1-x-yGexCy alloys grown by solid source molecular beam epitaxy on (100) Si substrates. X-ray photoemission spectroscopy was used to measure the valence band energies with respect to atomic core levels, and showed that C increased the valence band maximum of SiGeC by +48 meV/%C. The bandgap energies were obtained from optical absorption, and were combined with the valence band offsets to yield the conduction band offsets. For SiGeC/Si heterojunctions, the offsets were typically 0.6 eV for the valence band and 0.38 eV for the conduction band, with a staggered type II alignment. These offsets can provide significant electron and hole confinement for device applications.