A simple molecular model representing a solid ionic medium in equilibrium with a salt solution is described. Solid ionic media are permeable ion-exchanging bodies, e.g. ion-exchange columns, soil aggregates, certain shales, and ion-exchange membranes. The theory is particularly adapted to the latter.The transport processes caused by pressure, electrical and osmotic forces are described in terms of the volume concentrations of ions, water, and solid, and certain " friction coefficients " between these components. This analysis suggests approximations which reduce the number of the experiments needed for describing the transport processes. Application of the theory leads to relations of transport phenomena in a series of membranes of similar chemical nature, but different capacity and water content. This is illustrated by two sets of numerical examples where the theory is applied to data of other authors for cross-linked polyniethacrylate and sulphonated polystyrene membranes respectively.The model leads to flux equations which are consistent with the postulates of steadystate thermodynamics. In addition to the known laws of electrokinetics, permeability and electrical conductance, the theory also leads to a number of hitherto untested relationships.* This example has been chosen in order to avoid the complications caused by the presence of water. At 500°C sodium fluoride is a pure cationic conductor.10