We investigate the influence of carbon on the optical transitions ofSi1−yCyandSi1−x−yGexCylayers grown pseudomorphically on a Si(001) substrate also including full strain compensation. The layers were investigated by spectroscopic ellipsometry and electroreflectance spectroscopy for carbon fractionsy≤1.2 at. % and germanium fractions up tox=16 at. %. The spectra were analyzed by measuring and fitting electroreflectance spectra at 80 K and ellipsometry data at room temperature, resulting for both techniques in a weak and nearly linear dependence on the carbon fraction at all transitions. The results of both techniques are compared and discussed. The strong line broadening for increasing carbon fractions can be caused by a high scattering efficiency of the carbon. Our results indicate that the interpretation of optical spectra of carbon‐containing alloys cannot be performed straightforwardly by simple interpolating between the appropriate band structures of silicon, germanium, and carbon. An analysis based on strain‐induced contributions only also does not describe the experimental results correctly. For a complete description of the observed energy shifts detailed band structure calculations and further experimental data are necessary. ©1996 American Institute of Physics.