Metal separations from various mixtures of alkali metal, alkaline earth metal, Cu2+, Zn2+, Ag+, Tl+, and Pb2+nitrates were studied using an emulsion membrane system. The membrane consisted of a water-in-oil emulsion composed of 0·050 M LI4P2O7In H2O and 0·020 M dicyclohexano-18-crown-6 (DC18C6) in toluene with sorbitan monooleate serving as surfactant. The emulsion was placed into an aqueous source phase solution of the metal nitrates of interest. Of the cations studied, Pb2+was transported most rapidly and selectively. The selectivity of the system for particular cations is governed by the relative Mn+-DC18C6 and Mn+-P2O74−complex stabilities. Formation of a sufficiently stable Mn+-DC18C6 complex is necessary to partition cations into the toluene membrane, and formation of a more stable Mn+-P2O74−complex is necessary to strip cations from the membrane into the receiving phase.