Realization of group IV heterostructure devices requires the accurate measurement of the energy band offsets in Si/Si1-xGex and Si/Si1-x-yGexCy heterojunctions. Using admittance spectroscopy, we have measured valence-band offsets in Si/Si1-xGex heterostructures and conduction-band and valence-band offsets in Si/Si1-x-yGexCy heterostructures grown by solid-source molecular-beam epitaxy. Measured Si/Si1-xGex valence-band offsets were in excellent agreement with previously reported values. For Si/Si1-x-yGexCy our measurements yielded a conduction-band offset of 100 +/- 11 meV for a n-type Si/Si0.82Ge0.169C0.011 heterojunction and valence-band offsets of 118 +/- 12 meV for a p-type Si/Si0.79Ge0.206C0.004 heterojunction and 223 +/- 20 meV for a p-type Si/Si0.595Ge0.394C0.011 heterojunction. Comparison of our measured band offsets with previously reported measurements of energy band gaps in Si1-x-yGexCy and Si1-yCy alloy layers indicates that the band alignment is type I for the compositions we have studied and that our measured band offsets are in quantitative agreement with these previously reported results.