We use an analytical technique, previously shown to be able to decouple germanium and boron contents, to measure simultaneously the germanium and carbon contents of a set of carbon-doped epitaxial SiGe films. The method is based on the use of "perturbation functions" to account empirically for the effects that the dopant has upon the fundamental dielectric functions of the undoped material. For a set of five wafers with nominally the same germanium fraction but varying carbon levels, the ability to match measurements by secondary ion mass spectrometry using a production-grade optical metrology tool was demonstrated. Not only could correct results be obtained for the carbon content, but the agreement of the measurement of germanium fraction was also significantly improved. The method holds great promise for improving run-to-run process control for advanced epitaxy processes. (c) 2005 American Vacuum Society.