The solvent extraction of uranium(VI) and thorium(IV) with a tetra-carboxylated calix[4]arene (LH4) in chloroform has been studied in the presence or absence of alkali ions (M+=Na+, K+). When studied alone, UO22+and Th4+were extracted into chloroform as 2:2 and 1:1 metal:ligand complexes, respectively. The efficiency of extraction increases in the presence of alkali ions, due to the formation of heteronuclear complexes. For uranium(VI), the extracted species are found to be both 2:2:2 and 1:1:1 (uoi22+:M+:LH4,) mixed complexes. For Th(IV) in the presence of Na+, the formation of a mixed complex in 1:1:1 (Th(IV):Na+:LH4,) proportions has been evidenced. However, the exact nature of this species could not be determined. In practical grounds, LH. may be useful as a selective extracting agent for Th(IV) with respect to U(VI) since separation factor Th(IV)/U(VI) close to 1000 have been measured in competitive extraction, in the presence or absence of alkali ions.