For the design of Si1-xGex optical waveguide devices, one of the most important material parameters is the refractive index difference, delta n, between the alloy layer and the silicon substrate. We have measured delta n for 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 mu m. For transverse electric modes, the measured delta n varied with composition as delta n = (0.34+/-0.05)x. Transverse magnetic modes were more tightly confined to the waveguide layer and the index was determined to be delta n = (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 -0.080x and +0.095x for light polarized parallel and perpendicular, respectively, to the substrate plane, consistent with experimental observations.