One monomeric calixcrown (1) and four calixcrown oligomers (2-5) were used as carriers in a bulk liquid membrane. All the carriers transported K+ selectively, the order of the transport rate being: the calixcrown amide oligomer 4>the calixcrown ether oligomer 2>the calixcrown ester oligomer 3>the calixcrown siloxane oligomer 5, and the K+/Na+ selectivity decreased in the reverse order. These results demonstrate that the transport rate and the selectivity of calixcrown oligomers are related to the type of second component linking the calixcrown unit. In two-cation transport experiments, at high [K+]/[Na+] ratios, all the carriers transported K+ over Na+. As the ratio decreased, the transport of Na+ becomes favored, indicating that the cation flux depends, in part, on the relative concentrations of the cations in the source. Under our experimental conditions, the transport rate of Na+ remains almost unchanged with a constant concentration of Na+ and various concentration of K+. (C) 2002 Elsevier Science B.V. All rights reserved.