For the design ofSi1−xGexoptical waveguide devices, one of the most important material parameters is the refractive index difference,δn,between the alloy layer and the silicon substrate. We have measuredδnfor pseudomorphic waveguide layers with germanium fractions between 1% and 9% by fitting measured mode profiles to theoretical mode shapes for a wavelength of 1.3 μm. For transverse electric modes, the measuredδnvaried with composition asδn=(0.34±0.05)x.Transverse magnetic modes were more tightly confined to the waveguide layer and the index was determined to beδn=(0.55±0.05)x.The large difference between the two polarizations results from strain-induced birefringence. Bulk photoelastic theory, using constants appropriate for pure silicon, predicts strain contributions to the index of−0.080xand+0.095xfor light polarized parallel and perpendicular, respectively, to the substrate plane, consistent with experimental observations.