The interstitial carbon impurity (CI) vibrational modes in monocrystalline Si-rich SiGe were investigated using Fourier transform infra red (FTIR) spectroscopy and density-functional modeling. For less than 4.4% Ge concentrations, the two FTIR absorption bands of CI are found to be close to those in silicon, showing shifts in opposite directions with increasing Ge content. The transversal mode band at 932 cm−1 shifts slightly to the high-frequency side, while the longitudinal mode at 922 cm−1 suffers a pronounced red- shift. Each CI-related band is found to consist of two components, and most likely correspond to different combinations of Si and Ge atoms in the neighborhood of the carbon atom. The interstitial carbon defect was modeled by a supercell density-functional pseudopotential method (AIMPRO) for alloys with 4.69% Ge concentration. From energetics, it has been found that each Ge–C bond costs at least 0.4 eV in excess of a Si–C bond. However, structures where Ge atoms are second neighbors to the C atom are marginally bound, and may explain the two-component band structure in the absorption measurements. The vibrational mode frequencies taken from several randomly generated SiGe cells produce the observed opposite shifts for the transverse and longitudinal modes. Centers with high formation energy involving Ge–C–Si and Ge–C–Ge units producing frequencies, were not seen experimentally.