We report a detailed study of the dependence of the vibrational modes in rapid thermal chemical vapor deposition grown Si1-x-yGexCy samples on substitutional carbon concentration. The difficulty in incorporating substitutional carbon in these alloys with concentrations higher than a few tenths of an atomic percent is well known. We show that simultaneous analysis of x-ray diffraction, infrared absorption, and Raman scattering measurements can be used to determine the fraction of substitutional carbon from the total carbon content. We claim that the uncertainty in defining this fraction led to the discrepancies between the interpretations of previous studies. The frequency shifts of the C local modes and the Si–Si mode are studied, in addition to the intensity dependence of the former. The evolution of C satellite peaks and their dependence on total C concentration are presented. There is a large frequency change of these modes, indicating a very large local bond distortion induced by the presence of carbon in the lattice. We show that the shifts of the Si–Si mode frequencies due to the Ge and C content are not simply the addition of each contribution separately. The rate at which they shift with changing C concentration depends on the Ge content in the alloy. The probability of creating ordered substitutional C–C pairs, rather than randomly distributed C atoms in the crystal lattice, increases nonlinearly as a function of the carbon content. © 2001 American Institute of Physics.