Nanoscopic NaCl was prepared by elimination from sodium malonate and phenacyl chloride with a particle diameter of 100 nm. The NaCl crystals were added to either poly(D,L)lactide or polysulfone (dissolved in dichloromethane) as water-soluble porogens. The dispersion was applied to silicon wafers by dip-coating. After drying in air, the NaCl crystals were removed by washing with water, leaving behind a porous membrane with pore diameters around 100 nm. These free-standing membranes were prepared with a thickness from 1 to 10 μm. They can be used to separate small molecules from larger molecules like proteins.