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Hexacarboxylate derivatives of p-t-butylcalix [6]arene (2b) and p-hexylcalix[6]arene (3b) extract uranyl ion (UO;+) efficiently and selectively from water into organic media. This high selectivity can be attributed to the nature of calix [6]arene; this is suitably structured for the binding of U02'+, which requires a pseudoplanar h exacoord i nation . When t r i octy I met h y I a m mo n i u m cation (T M A + ) is added to the organic phase, the stoicheiometry of the extraction species [U02'+/(2b)] changes from 1 : 3 t o 1 : 1-2. This implies that an additional 1-2 mol of UO,'+ are adsorbed to (2b) t o form 'neutral' extraction species and can be displaced by added TMA'. UO, " is transported by (2b) across a liquid membrane from a neutral IN (source) aqueous phase (pH 8.5) to an acidic OUT (receiving) aqueous phase (0.1 mol dm-3 HCI) in an active transport manner. The rate-determining step is the release of UO,*+ from the membrane to the OUT aqueous phase. On the basis of these basic data, w e have prepared polymer/liquid-crystal (N-4'-ethoxybenzylidene-4-butylaniline, EBBA/TMA+CI-) composite membranes immobilising (2b) and have found that the rate of UO, , ' transport is efficiently controlled by the changes in pH and temperature. These are novel examples for the selective transport of UO,'+.