The complex refractive index of SiGe alloys at 632.8 nm has been measured as a function of the Ge concentration usinginsituellipsometry while the material is slowly removed from a silicon substrate using reactive‐ion etching (RIE). Homogeneous, strained epitaxial SiGe films on silicon substrates were used. The Ge concentration was obtained by Rutherford backscattering. If an unknown SiGe structure is etched with RIE,insituellipsometry yields combinations of the ellipsometric angles &PSgr; and &Dgr; with time. Starting at the Si substrate, these points are, on a point‐to‐point basis, converted into combinations of complex refractive index and depth in a numerical procedure. For this inversion of the ellipsometry equations, the known relation between the real and the imaginary part of the refractive index of SiGe is used. Finally the refractive indices are converted into Ge concentrations. Thus the depth profile of the Ge concentration in an unknown epitaxial SiGe structure can be inferred from aninsituellipsometric measurement during RIE of the unknown structure. The obtained resolutions in depth and Ge concentration are 0.3 nm and 0.3%, respectively.