We have used admittance spectroscopy to measure conduction-band and valence-band offsets inSi/Si1−xGexandSi/Si1−x−yGexCyheterostructures grown by solid-source molecular-beam epitaxy. Valence-band offsets measured forSi/Si1−xGexheterojunctions were in excellent agreement with previously reported values. Incorporation of C intoSi1−x−yGexCylowers the valence- and conduction-band-edge energies compared to those inSi1−xGexwith the same Ge concentration. Comparison of our measured band offsets with previously reported measurements of energy band gaps inSi1−x−yGexCyandSi1−yCyalloy layers indicate 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. ©1997 American Institute of Physics.