We report on the investigation of thin, epitaxially grown S1-xGex layers on Si(001) substrates by spectroscopic ellipsometry over the entire composition range. S1-xGex layers with Ge fractions between 17 and 100% at a fixed layer thickness of similar to 100 nm were grown on 100mm Si(001) p-type wafers using surfactant-mediated molecular beam epitaxy with Sb as a surfactant. The degree of strain relaxation increases with Ge fraction, it varies from fully strained layers for low Ge fractions to almost complete relaxation for high Ge fractions. A parameterization is being presented with which the Ge content and layer thickness of thin SiGe layers on Si(001) substrates can be determined. We find a good correlation between the obtained results from the ellipsometry measurements and X-ray diffraction and X-ray reflectometry results, which are used as a reference for sample characterization. We observe that the E1 interband transition in the band structure of SiGe layers changes almost linearly with the Ge fraction for our samples. We use a modified Lorentz oscillator model to fit the measured curves, using six oscillators for high Ge fractions (x>0.59) and four oscillators for low Ge fractions (x<0.59). (C) 2016 Elsevier B.V. All rights reserved.