The cathodoluminescence emission spectrum of Yb³⁺ in cadmium sulphide at 10 K has been studied before and after irradiation with electrons capable of producing atomic displacements. The spectrum has been assigned to emission from Yb³⁺ on a weakly distorted tetrahedral site such as it would be if the Yb³⁺ were replacing a cadmium ion. Multiplets with the main groups of lines are believed to result from the influence of sulphur ions, possibly at nearby interstitial sites. Electron radiation damage above the threshold energy for sulphur displacement, and with the sample at a temperature ( approximately 320 K) at which the defects are sufficiently mobile, causes a redistribution of sulphur amongst the interstitial sites which changes the relative intensities of the rare earth emission lines.