The complexation behaviour between salts of Li+-Rb+ in CD3CN and tris(benzocrown ether)s 2a,b X=P(NMeN=CHB15C5) 3 (X = O, S) and tri[bis(benzocrown ether)] [N=P(NMeN=CH-B15C5)2]3 3 was investigated by 13C NMR spectroscopy. Using the program RMNSTAB, the complexation constants for the different possible complexes (M2L, ML and ML2 were L represents one benzo-15-crown-5) were obtained and were compared with those of the corresponding monomer material. A remarkable “biscrown effect” for compounds 2a,b and 3 was found, especially for potassium and rubidium by the predominant formation of stable ML2 complexes. The strong chelate effect make these ligands highly efficient extracting agents for alkali metal picrate salts of K+, Rb+ and Cs+, as shown by UV-Vis spectroscopy.