AbstractThe finding that dispersions of insulators such as alumina or silica in ionic conductors may exhibit considerably enhanced ionic conductivities compared to the pure material, has become a promising topic for optimizing the electric properties in solid state research. Up to now the basic mechanism for this effect is unknown, the more so since two phase mixtures of two ionic conductors have been found to show a similar enhancement. In this work space charge regions are considered as being responsible for the effect. In the case of insulators as second phases possible surface interactions are taken into account resulting in drawing mobile ions out of the boundary layers and thus increasing the vacancy concentration or in pushing mobile ions into interstitial positions. In the case of two ionic conductors with the same mobile ion, the contact equilibria demand a double effect. The concentration distributions are discussed and lead to a quantitative expression for the space charge contribution in such dispersions. These calculations are in excellent accord with conductivity data of silver chloride (+ alumina, silica) as regards temperature, concentration and particle size. Experiments from the literature are also brought into the discussion. The influence of the surface activities of alumina is tested by modifying the surface chemically. Thermodynamic data for the surface interaction are given. The maximum effect to be expected by conductivity enhancements via surface induced defects and optimization conditions are discussed.