The influence of carbon on optical transitions in SiGeC layers grown pseudomorphically on Si(100) substrate by rapid thermal chemical vapor deposition was investigated. We used spectroscopic ellipsometry to extract the pseudodielectric function for two sets of layers, with Ge-content of 8 and 18% and with C-content between 0 and 1.8%. From the numerical derivatives of the measured dielectric functions, we determined the critical point energies E-1, E-1 + Delta(1), and E-2 as a function of the C-content. A weak linear dependence on the C-content was observed for all transitions. The experimental results could not be described either by considering solely the alloying effect, nor by linear addition of the strain-induced contributions. For a complete description of the observed energy shifts, a detailed band structure calculations and further experimental data are necessary.