Since the index of refraction of AgClxBr1-x (x < 1) is higher than that of AgCl, by diffusing Br ions into AgCl it was possible to control the index and thus obtain planar waveguides made from silver chlorobromide (AgClBr) on a AgCl substrate. Silver halides are transparent in the mid IR, and it was therefore possible to characterize the waveguides by transmission of 10.6-μm CO2-laser radiation through them. In a typical case, the thickness was optically measured and was found to be 65 μm, and the propagation loss was 16 dB/cm. The output-beam profile distribution was determined experimentally and found to be well correlated with a numerical analysis simulation based on a ray-tracing model of the eikonal equation. Planar waveguides that are transparent in the mid IR will likely be useful in numerous applications.