The extent of interdiffusion in Cd1−xMnxTe/CdTe quantum‐well structures grown by molecular‐beam epitaxy was monitored by photoluminescence and photoluminescence excitation spectra. Thermal annealing of as‐grown and ion‐implanted structures in over pressures of cadmium (or tellurium) provide clear evidence that diffusion is controlled by cation vacancies and are consistent with a strong dependence of the interdiffusion coefficient on the vacancy concentration. ©1996 American Institute of Physics.