We report a novel type of inorganic membranelike structure formed by the self-assembly of hydrophilic polyoxometalate macroanions. Such nanoscaled, water-soluble macroions tend to form stable, uniform, single-layer "blackberry" structures (20-1000 nm in size) in dilute solutions via noncovalent bond interactions. Two interesting features of "Keplerate" {Mo72Fe30} blackberries are found from fluorescence studies: (1) They create a microscaled, relatively isolated water environment (containing over 3 million water molecules) which possesses different properties from the bulk water. (2) The blackberry membrane is permeable to small cations, but not to anions. The passive transport of cations across the blackberry membrane is relatively slow but does not need any carrier or additional energy.