The characteristic electrochemical behavior of membranes, especially their electromotive action, has recently been elucidated on the basis of their being ion exchangers. New methods are developed for the preparation of electronegative, selectively cation‐permeable, and electropositive, selectively anion‐permeable, membranes of various degrees of porosity, including “permselective” membranes which act in concentration chains as virtually ideal reversible machines for the transfer of ions, either of cations or anions as the case may be. The electrochemistry of these membranes is described in detail. Permselective membranes can be used as “membrane electrodes” for the electrometric determination of the activities of practically all univalent anions and cations, for the study of Gibbs‐Donnan membrane equilibria in systems containing only low‐molecular‐weight strong electrolytes, for the study of mixed electrolyte solutions and many kindred problems, and in various ion exchange processes between solutions.