Experiments have been carried out on As diffusion in graded-band-gap HgCdTe epitaxial layers grown by isothermal vapor-phase epitaxy (ISOVPE). For studying As and Hg concentration distribution on depth, the SIMS experimental technique was used. A non-monotonous profile of As concentration distribution has been revealed, with the maximum distribution approximately located in the region at the edge of a sharp change in composition. A model describing the influence of the built-in electric field caused by the spatial inhomogeneity of energy gap on the diffusion of the charged impurity has been proposed. A comparative analysis of theoretical and experimental data has been performed. (C) 2003 Elsevier B.V. All rights reserved.