Systems of crystalline ionic solutions containing impurity cations and associated compensation defects are described. Interactions between aliovalent ions and compensation defects give rise to configuration partition functions which predict a distribution in ion-defect pair separation distances at low temperatures. At elevated temperatures such ion-defect pairs dissociate, and the concept of pair formation more appropriately gives way to the concept of pair correlation functions. The relative sizes of the aliovalent ion and the host ion which it replaces are seen to exert a pronounced effect on pair distribution. Salient features of the theory are applied to the KCl: Sr2+, NaCl: Mn2+, NaCl: Sr2+, LiCl: Mn2+ and (alkaline-earth halide): (rareearth)3+ systems. While the high-temperature treatment lends itself to comparisons with the Debye-Hückel theory for electrolytes, the low-temperature pair distribution theory is shown to be valid chiefly because of the specific nature of the crystalline ionic solutions
