Enough experimental data on nonaqueous ion-exchange separations have been gathered to show that one is dealing with a field whose theoretical aspects are infinitely complicated. The separations can be based on several different types of chemical reactions. Apart from true ion exchange, sorption can take place by non-exchange reactions such as acid-base reactions or complex-formation, either by hydrogen bonding or by metal-ligand coordination. All these reactions can be influenced by a multitude of parameters such as solvent polarity, solvent-solute interaction by solvation or complex formation, solvent structure changes, solvent-resin matrix interaction, solvent-solvent interaction, acid-base properties of the solvent, non-exchange electrolyte invasion, and so on. Any one or several of these parameters may or may not be operative in a given case, and a given parameter can be of primary importance in one case and be only a minor contributing factor in another. In view of such complexity, one should not be surprised that the few attempts so far made to develop a theory of nonaqueous ion exchange were not too successful and could be applied only to a few rather simple cases. Even considering only one type of chemical reaction, the true ion exchange, the chances of ever arriving at a comprehensive theory appear to be pretty slim.