The high-temperature conductivity (HTC) relaxation in CdS and CdSe single crystals, caused by a steep change of cadmium vapor pressure PCd, was measured. For obtaining a steep change of PCd improved van Doorn equipment was used. Conductivity measurements were carried out using the Van der Pauw method. It was shown that the Cd diffusion from gaseous phase into CdS and CdSe platelets leads to a conductivity time-dependence function σ( t) in the form of a sum of decreasing exponents, indicating that the rate of HTC relaxation is limited by the chemical diffusion of cadmium. A correlation between chemical and tracer-diffusion coefficients was found. Values of activation energies and preexponential factors for both types of diffusion were determined. The results are discussed using a model for transformation of Cd ··i into V ··S,Se previously proposed by Chern and Kröger for chemical self-diffusion in CdTe.