The diffusion profile of Hg in CdTe crystals was observed by means of heavy ion (40 MeV O5+) backscattering. The near‐surface region of CdTe immersed in Hg was investigated up to 1.4 &mgr;m from the surface. The observed Hg profile indicated that the concentration of Hg atom at the surface reached 4×1020cm−3and the distribution was interpreted by a simple diffusion model. Temperature dependence of the diffusion coefficient was determined to beD=5×103 exp[(−2.0±0.3) eV/kT] (cm2/s). In the case of CdTe immersed in Hg which contained a small amount of Cd, it was found that Hg diffusion did not occur. These experimental facts suggest that the rate of Hg diffusion is controlled by the diffusion of a Cd vacancy which is introduced by the outdiffusion of Cd atoms from CdTe crystals.