Abstract The II–VI semiconductor cadmium mercury telluride has been studied with a low-energy positron beam. Differences in the variation of the Doppler line-shape parameter as a function of positron implantation energy have been observed for annealed and as-grown samples. A diffusion model analysis of the results indicates large changes in the defect concentration in the bulk due to the annealing. This change is attributed to the difference in mercury vacancy concentration in the samples.