Local vibrational modes of carbon impurities in relaxed Si 1Ϫx Ge x have been studied with infrared absorption spectroscopy in the composition range 0.05рxр0.50. Carbon modes with frequencies in the range 512-600 cm Ϫ1 are observed in 13 C ϩ -implanted Si 1Ϫx Ge x after annealing at 550°C. Measurements on samples coimplanted with 12 C ϩ and 13 C ϩ show that these modes originate from defects containing a single carbon atom and from the variation of the mode frequencies with composition x, the modes are assigned to substitutional carbon in Si 1Ϫx Ge x . Based on the frequencies obtained from a simple vibrational model, the observed modes are assigned to specific combinations of the four Si and Ge neighbors to the carbon. The intensities of the modes indicate that the combination of the four neighbors deviates from a random distribution. Ab initio local-densityfunctional cluster theory has been applied to calculate the structure and the local mode frequencies of substitutional carbon with n Ge and 4Ϫn Si neighbors in a Si and a Ge cluster. The calculated frequencies are ϳ9% higher than those observed, but the ordering and the splitting of the mode frequencies agree with our assignments.