Copper was diffused in cadmium sulfide single crystals to determine diffusion coefficients using the radio‐active tracer sectioning technique. Over the temperature range of 450°–750°C, the relationship between temperature and diffusion coefficient was found to beD=1.5×10−3e−17600/RTsquare centimeters per second for diffusion perpendicular to theCaxis. For diffusion parallel to theCaxis, the frequency factor and activation energy were found to be within the experimental accuracy of the corresponding values for perpendicular diffusion. There appears to be no appreciable diffusion anisotropy. Observed surface diffusion was a complicating factor in obtaining data on volume diffusion.