The complex dielectric functions Є(ω) from 0.75 to 6.6 eV of pseudomorphically strained Si1-xGex (0≪x≪0.275) and Si1-x-yGexCy (x≈0.21, 0≪y≪0.013) alloys grown on Si (001) were determined using spectroscopic ellipsometry. Our rotating-analyzer instrument uses a computer-controlled MgF2 Berek waveplate as a compensator to improve the accuracy of the ellipsometric angles, particularly below 3 eV. By performing a least-squares analysis of the raw data, taken at three angles of incidence, we obtain the thicknesses of the alloy and the native oxide cap as well as Є(ω) for the alloy, which is parametrized using a semiempirical oscillator model. Differences between our data and those in the literature are due to differences in strain conditions and/or the improved accuracy of our instrument employing a compensator, which allows the determination of the native oxide thickness from measurements at long wavelengths. We apply our dielectric functions to analyze a variety of group-IV heterostructures and find good agreement with high-resolution x-ray diffractometry. The optical constants reported here are important, because they allow thickness and composition measurements using automated inline spectroscopic ellipsometers and reflectometers. © 2000 American Institute of Physics.