AbstractAqueous regions within cationic permselective membranes were filled by cathodically deposited silver and sections made of this material were observed in the electron microscope. The silver itself deformed the matrix and crystallized in large globules, but the background of the micrographs, the Ag form of the membrane, showed characteristic structural elements. Anionic membranes repeatedly impregnated with telluride ions and oxydized by air also contained granular tellurium metal, which evaporated during exposure to fast electrons in the electron microscope. Using only the sulphonium groups of the membranes or heavy counter ions as electron absorbing material, we examined the structure of some membranes. Those made of polyethylene reveal a series of black and white lines, often arranged in black‐white‐black triplets, which probably are cross sections of crystalline platelets of polyethylene, reacted on their surface. Those made of PVC reveal large areas which underwent substitution to a lesser extent, surrounded by more strongly reacted coherent material. Other membranes also show characteristic elements in their microstructure.