Refractive index profiles resulting from the fabrication of optical waveguides by diffusion techniques are measured using a reflection interferometric technique. In Cd-diffused ZnSe waveguides, the index variations are found to be complementary error functions that closely follow the composition changes for deep ( greater than 5 mu m) diffusions. Shallow ( less than 5 mu m) diffusions produce waveguides in which the index profile is a complementary error function that differs significantly from the composition profile. The relationship between composition and refractive index is determined for Cd compositions less than 10%. Refractive index profiles in commercially available diffused glass waveguides (SELFOC rod and fibers) are also described.