The tracer-sectioning technique was used to study the diffusion of aurous ion in silver chloride. Above 260°C, the diffusion proceeds interstitially with an activation energy of 0.47 eV. When the diffusion anneal was carried out in an external chlorine atmosphere, the diffusion coefficient was depressed. This behavior is explained in terms of the effect of chlorine on the cation vacancy concentration, and agrees quantitatively with results obtained from optical absorption studies by Ulrici, Koswig and Stasiw. Analysis of the data yields 0.62 eV for the heat of the reaction: 1 2Cl2 = Cl- + cation vacancy + hole. © 1969.