Polymerizing pseudo-crown ethers in the pores of a 70 mu m thick polyethylene membrane support produced polymeric fixed site carrier membranes for facilitated transport separations. Pseudo-crown ethers were synthesized from the photopolymerization of metal : poly(ethylene glycol 200) diacrylate (PEG200DA) complexes. The metal:monomer complex led to a near-circular conformation of the monomer in which the two reactive endgroups were in close proximity; thus, the probability of intramolecular cyclization (i.e. pseudo-crown ether formation) was increased. Facilitated transport membranes using pseudo-crown ethers as carrier species were selective for alkali metals over transition and lanthanide metals. The K over Na selectivity was as high as 1.5, while the selectivity of K over Nd3+ was as high as 3700. The flux and selectivity of metal ions were a function of carrier concentration and type as well as the cross-linking of the pseudo-crown ether network.