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Using the nonequilibrium thermodynamic treatment of Kedem and Katchalsky a study has been made of series arrays of ion‐exchange membranes. A rule has been found which identifies certain classes of intermembrane concentrations which vary in the same way with respect to current under different steady‐state conditions. Among these is a set which is independent of the value of the steady‐state current. For the special case of series arrays containing alternately positive and negative fixed charges, with indefinitely large end reservoirs, it is shown that in the steady‐state the array may possess a set of desalinated intermembrane compartments, or a set of salinated intermembrane compartments, but never both simultaneously. This leads to the conclusion, which has been tested experimentally, that a large array of this kind may act as an efficient rectifier. The steady‐state concentration profile across a large array may be radically altered by changing a single membrane in the array, or even adding a single membrane at one end of the array. The approach to the steady‐state has been examined by means of a relaxation calculation, which leads to certain conclusions relating to optimization of flow‐rates in desalination by electrodialysis.

The behavior of precipitate‐membranes of BaSO4 in cellophane is discussed in terms of the formation process of the system and the known permeability to Ba++, SO=4 and to water components. It is shown that the available information leads to an interpretation in terms of an electrical bipolar model. The specific case of Ba(OH)2 and H2SO4 solutions separated by the above membrane is discussed in detail. The rectification effect and the non‐zero stationary currents observed are discussed in terms of the flows of H+, OH− and the flow of the chemical reaction of water formation. The logarithmic term derived for the voltage current relationship is shown to be directly linked with the depolarizing effect of water formation, and thus quantitatively related to the kinetic constants of water formation and dissociation.

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