Ultrathin separation membranes were prepared upon alternating electrostatic adsorption of 1,4,7,10,13,16-hexaazacyclooctadecane (18-azacrown-6, hexacyclen, aza6) and polyvinylsulfate on porous polyacrylonitrile/polyethylene terephthalate (PAN/PET) substrates. The resulting composite membranes were highly permeable for electrolyte solutions, but the selectivity in ion transport was poor (alpha(Cl-/SO42-) = 2). However, after a treatment of the membrane with 0.1 M aqueous copper(II) acetate solution, the rejection of divalent anions was strongly enhanced. Separation factors alpha(Cl-/SO42-) = 110 and alpha(Cl-/SO32-) = 1420 were found. Spectroscopic studies indicate a highly specific complex formation of aza6 and copper acetate in the membrane. Treatment of the membranes with cobalt(II) and nickel(II) acetate also led to an enhancement of the separation factors, but the effect was much smaller. Detailed studies of the influence of the pH of the dipping solutions and number of deposited bilayers are also reported. A model is presented describing the origin of the high transport selectivity.