The defect chemistry in boundary regions of ionic crystals is considered from a thermodynamic point of view. The related effects on the ionic conductivity of two‐phase mixtures and other heterogeneous systems are discussed. Experimental and theoretical findings are critically reviewed. The concept of the formation of extra defects in the space charge regions as a consequence of interface processes (internal charge transfer) is shown to be a reasonable explanation for many effects on the electric conductivity which are observed in two‐phase systems involving ionic conductors (heterogeneous doping) as well as in some pure polycrystalline materials. The concept is extended to the problem of thin films and to the consideration of concentration distribution and partial conductivities of the electronic minority charge carriers.