Infrared waveguides have been made by isothermal evaporation diffusion of HgTe on a ZnTe substrate and characterized at 10.6 μm by m-lines analysis. Refractive index profiles deduced from this analysis are correlated to the composition ones measured with a microprobe. As a result, we obtain the refractive index of the alloy as a function of the zinc concentration ranging from 0.2 to 1. Direct measurement of the substrate refractive index gives a value 2.60 to 2.65, results in slight disagreement with the previous data.