Adsorption of diaza-18-crown-6 ether substituted with two tetracosane (-C24H49) alkyl chains (ACE-24) was investigated at the liquid-liquid interface. X-ray reflectivity measurements determined the structure of a close-packed monolayer at the hexane-water interface, which is consistent with conclusions drawn indirectly from earlier interfacial tension measurements on similar molecules. These data provide further insights into the role of interfacial processes involving azacrown ethers in ion separation techniques such as the permeation liquid membrane. (C) 2010 Elsevier B.V. All rights reserved.