We have used, admittance spectroscopy and deep-level transient spectroscopy to characterize electronic properties of Si/Si1-x-yGexCy heterostructures. Band offsets measured by admittance spectroscopy for compressively strained Si/Si1-x-yGexCy heterojunctions indicate that incorporation of C into Si1-x-yGexCy lowers both the valence- and conduction-band edges compared to those in Si1-xGex by an average of 107+/-6meV/%C and 75+/-6meV/%C, respectively. Combining these measurements indicates that the band alignment is type I for the compositions we have studied, and that these results are consistent with previously reported results on the energy bandgap of Si(1-x-y)Ge(x)C(y )and with measurements of conduction band offsets in Si/Si1-yCy heterojunctions. Several electron traps were observed using deep-level transient spectroscopy on two n-type heterostructures. Despite the presence of a significant amount of nonsubstitutional C (0.29-1.6 at. %), none of the peaks appear attributable to previously reported interstitial C levels. Possible sources for these levels are discussed.