The role of Ag+‐interstitial and Ag+‐vacancy transport across silver halide surfaces is important for both ionically blocking and nonblocking interfaces. One technique to evaluate the nature of a blocking silver halide surface is to measure the alternating‐current dielectric loss of silver halide microcrystals in contact with an insulating dry‐gelatin phase. From the results of these measurements, the availability of the potential‐determining Ag+interstitial was determined as a function of silver halide type, crystal size and morphology, and the addition of divalent cations within the silver halide phase and adsorbed at the surface. The results on small crystals (0.2 &mgr;m) and, analogously, for very thin films, where the Debye length is comparable to the linear thickness, indicated that the Ag+‐interstitial concentration is a critical function of the surface properties of the silver halide. Consequently, the transport of ionic carriers across the silver halide is not only dependent on bulk solid‐state properties, but also on the nature of the surface and the ionic space‐charge layer just below the surface.