Electron microscopic and electron diffraction studies have been carried out on cupric ferrocyanide gels, chosen as a prototype of an inorganic semi-permeable membrane. It was believed that a study of the structure and morphology of such membranes, which are seemingly less complicated than the important organic semi-permeable membranes surrounding all living cells, will throw light on the little understood phenomenon of semi-permeability. It has been found that cupric ferrocyanide gels consist of loosely and randomly packed primary particles in the size range of 10 to 30 A. The pore space is the interstices between the particles. The primary particles are crystalline, the electron and x-ray diffraction patterns corresponding to the standard pattern of Cu2Fe(CN)6. However, samples prepared and washed very rapidly exhibit more disorder, and consist of extremely small and imperfect crystals, or may be essentially amorphous, in the sense that liquid type diffraction patterns are observed. Aging under moderate conditions results in an increase in crystal size to the range of 30 to 200 A. This observation may explain why many earlier investigators aged and washed for a long time the practical cupric ferrocyanide membranes deposited in the pore space of a parchment thimble or porous clay cup, inasmuch as the film or membrane composed of large crystals may be stronger, and thus serves as a better semi-permeable membrane. In view of the small primary particle size and the small size of the interstices, it is concluded that among the classical theories of semi-permeability, the adsorption theory is predominant, but that solution and size effects play an important role.