The Hg concentration of anodic oxide films on Hg1−xCdxTe and its change after heat treatment were investigated by using 40 MeV‐O5+ backscattering. Heat treatment in a vacuum for 60 min was performed at various temperatures up to 350 and 400 °C, respectively, for the samples with x=0.19 and x=0.32. A significant Hg content in the as‐grown anodic oxide was found in accordance with the recent result of C. M. Stahle, D. J. Thomson, C. R. Helms, C. H. Becker, and A. Simmons [Appl. Phys. Lett. 47, 521 (1985)]. The Hg in the film gradually decreased with increasing heat treatment temperature up to about 290 °C and rapidly disappeared above 290 °C for both x=0.19 and x=0.32.