The addition of carbon atoms into Si or SiGe layers on Si opens the possibility for strain and bandstructure engineering. In this paper we will investigate the influence of carbon on the optical transitions of Si1−yCy and Si1−x−yGexCy layers grown pseudomorphically on Si(0O1) substrate using solid source MBE incluing also full strain-compensation. The layers were investigated by spectroscopic ellipsometry and electroreflectance spectroscopy for y ≤ 1.2 %. From the numerical derivatives of the measured dielectric constants, we determined the critical points energies E01, E1 and E2 as a function of the carbon content y. This shift was analyzed by measuring and fitting electroreflectance spectra at 80K and ellipsometry data at room temperature, resulting in a weak and nearly linear dependence on the carbon content at all transitions. These dependencies indicate that the interpretation of optical spectra of C-containing alloys can not be performed straightforward by simple interpolating between the appropriate band structures of silicon, germamum and carbon. An analysis based on only strain-induced contributions also does not describe the experimental results correctly. For a description of the observed energy shifts for pseudomorphic Si1−X−yGexCy we have to consider at least strain-induced effects and effects due to alloying.