Simplest ion exchange systems, containing two phases and three components (two electrolytes of the exchangeable ions and a solvent), are considered from the viewpoint of thermodynamics of mixtures. Specific of these systems is the presence at the ion exchange phase of a spatially infinite, osmotically inactive matrix carrying an electrical charge. Application of some important concepts and equations of chemical thermodynamics of mixtures, such as chemical potential, activity coefficients, Gibbs free energy, equations of Gibbs-Duhem and of the equilibrium constant, have been discussed. It is concluded that ion exchange is a process combining features of heterogeneous chemical reaction and osmotic distribution. It can be equally well described as a chemical reaction complicated by osmotic phenomena as vice versa, depending on the properties of the ion exchanger and the statements of the theoretical approach.