The synthesis of new P?O-disubstituted receptors with appended crown ethers and their properties as receptors for dicarboxylic acids have been studied. High affinities have been observed (oxalic and malonic acids with 4-, 5-, 6-, or 8-crown ethers). Binding of a cationic effector within the crown ether unit resulted in a positive “allosteric” effect, which has been determined to be K_rel = 7 in the best case (binding of malonic acid with Li⁺ @ rac-3b).