The solution structures of a linear synthetic carboxylic polyether ionophore, HO[CH2CH2O(1,2-C6H4)O](4)-CH2(1,2-C6H4)COOH, 1, and its alkali metal salts have been studied by H-1 and C-13 NMR spectroscopies (H-1-H-1 COSY, NOESY, C-13-H-1 COSY, HMQC, HMBC, and long range C-13 J resolved 2D-NMR). The ionophore 1 selectively transports K+, Rb+, and Cs+ over Na+ and Li+. The H-1 NMR spectra of 1 and the Li and Na salts are simple, whereas those of K, Rb, and Cs salts are more highly chemical shift resolved but are very similar to each other. Conformational analyses of K, Rb, and Cs salts were carried out on the basis of the 3D structures in the crystal using vicinal three-bond coupling constants, NOE, and chemical shifts, which depend on the torsion angles of 1,2-ethylenedioxy, the distances between methylene and aromatic methyne protons, and the magnetic anisotropy mainly resulting from the aromatic rings, respectively. These data show good correlation with those expected from the structures in the crystal. The backbone bending observed in the crystals was also confirmed with chemical shifts and (3)J(H-C) of C-13 NMR spectra of the K, Rb, and Cs salts. These results show that the conformation of K, Rb, and Cs salts which is similar to that in the crystal is dominant in a liquid membrane, but the Li and Na salts remain highly mobile in solution. These results are discussed in connection with the ion-transport ability of 1 through the liquid membrane.