The diffusion constantD(T) of cadmium into single‐crystal zinc selenide was measured using a photoluminescence/etch technique. Diffusions were performed in sealed ampoules using cadmium metal and cadmium selenide powder as dopant sources. For cadmium metal sourcesD(T) = 6.39 × 10−4exp(− 1.87 eV/k T) in the range 700–950 °C.