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A series of new calixarene derivatives with phosphine oxide groups on the lower rim and, for comparison, a series of noncalixarene phosphine oxides have been synthesised. Their extraction power for alkali and alkaline earth cations from aqueous metal picrate solution into dichloromethane have been determined as well as the stability constants in methanol of the 1:1 complexes of several members of the calixarene series. Important selectivity trends are revealed by both methods. The extraction power from aqueous nitrate solutions (1m in HNO 3 ) towards europium(iii), as a model for trivalent actinides, and thorium(iv), as a model for tetravalent actinides, has been studied in detail for eight symmetrical calixarene derivatives, which differ in either calixarene size (4, 5, 6 or 8), the substituent at the upper rim (tert-butyl or hydrogen) or the number of methylene groups separating the phenolic oxygen atoms from the phosphorus atoms (1 or 2). The stoichiometry of the extracted species was characterised by a classical log ± log plot analysis. All the calixarenes tested are far better extractants than typical noncalixarene extractants, for example, TOPO and CMPO, currently in use in the treatment of radioactive waste; they extract thorium better than europium. The influence of the nitric acid concentration and of the sodium nitrate concentration in the aqueous phase on the extraction efficiency was also examined in order to assess the possible application of these compounds for the decategorisation of liquid nuclear waste. Several thorium complexes have been characterised by their stability constants in methanol.

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