The influence of the extractant on cesium transport from nuclear fuel reprocessing concentrate solutions to demineralized water through flat-sheet-supported liquid membranes (FSSLMs) was studied using four crown ethers of different lipophility : dibenzo-21-crown-7 (DB21C7), benzo-21-crown-7 (B21C7), t-butylbenzo-21-crown-7 (tBuB21C7), and n-decylbenzo-21-crown-7 (nDecB21C7). The distribution coefficients of cesium show that the four crown ethers have similar extraction capacities which follow the sequence nDecB21C7 > tBuB21C7 > B21C7 >; DB21C7. DB21C7 and B21C7 cannot be used in cesium transport experiments due to the low organic solubility of the former and the great tendency of the latter to form a third phase between the organic phase and the aqueous extraction phase. Of the other two crown ethers, nDecB21C7 leads to faster cesium transport than tBuB21C7 due to the stronger lipophilic character of the former; this allows a higher crown-ether concentration in the membrane to be obtained when nDecB21C7 is used.