Abstract The solvent extraction of rubidium and caesium picrates has been studied between benzene and water at 25 °C in the presence of tributyl phosphate (B), 12-crown-4 (12C4), 15-crown-5 (15C5), or both B and 15C5, and the complex-formation constants for the synergistic reactions in the benzene solution have been calculated. For both rubidium and caesium, the sequences of the complex-formation constants for these ligands are B<12C4<15C5, indicating that the complex-formation constant increases with an increase in the number of the donor oxygen atoms of the ligand. The complex-formation constant of rubidium is greater than that of caesium in every system.