Abstract: The invention relates to new calixarenes of formula: ##STR1## in which R.sup.1 represents a crown ether chain that includes at least two aryl or cycloalkyl rings,R.sup.2 is a hydroxyl or alkoxy group, or the two R.sup.2 groups together form a crown ether chain such as R.sup.1, and R.sup.3 represents a hydrogen atom or an alkyl group. The calixarenes are used to selectively extract caesium from aqueous solutions that notably have high concentrations of sodium.

Abstract: The invention relates to bis-crown calix[4]arenes, their preparation process and their use for the selective extraction of cesium and actinides. Bis-crown calix[4]arenes are in accordance with the formula: ##STR1## in which R.sup.1 and R.sup.2 respectively represent X(C.sub.2 H.sub.4 X).sub.m and X(C.sub.2 H.sub.4 X).sub.n with X.dbd.O, NH and/or N(CH.sub.3), m and n=3, 4, 5 or 6, or X (C.sub.2 H.sub.4 X).sub.p/2 XY(C.sub.2 H.sub.4 X).sub.p/2 with p=2 or 4 and Y=cycloalkylene or arylene. These bis-crown calix[4]arenes, whose benzene nuclei are optionally substituted by alkyl groups, can be used as an extracant, e.g. in the form of a liquid membrane, for separating cesium from acid solutions containing sodium in a large quantity compared with the cesium quantity, e.g. effluents of irradiated fuel reprocessing plants.