In this paper it is shown that the optical determination of free‐carrier concentrationN0in 5–10 &mgr;m thick epitaxial layers ofn‐type silicon grown on N+or N−substrates is possible for concentrations ≥2×1016cm−3by measuringp‐polarized reflected lightRpnear the Brewster angle at a wavelength &bartil;10 &mgr;m. Good crystallinity, constant concentration profiles normal to the surface, as well as relatively small differences in the index of refraction with respect to the substrate are essential requirements in order to get high resolution. Good agreement with the angular derivative ofRpis obtained.