Abstract. The synergistic extraction of some divalent cations (Cd, Cu, Mn and Zn) in chloroform, with 3-phenyl-4-benzoylisoxazol-5-one (HPBI) in the presence of various synergistic agents: trioctylphosphine oxide (TOPO), octyl(phenyl)(N,N-diisobutylcarbamoylmethyl)phosphine oxide (CMPO) and bis(diphenylphosphinyl)methane (BDPPM) has been studied. It is compared with the extraction in toluene. Due to various factors (e.g. extractant-diluent or extractant-synergist interactions), the stoichiometry of the extracted species, the selectivity and the magnitude of the synergism depend on the diluent and on the synergist. In chloroform, the extraction with HPBI and TOPO follows the order Cu > Zn > Mn > Cd (Mn > Zn in toluene). In chloroform, the synergistic extraction of cadmium follows the order BDPPM > CMPO > TOPO (TOPO > CMPO in toluene). The formation constants of Cd(PBI) + and Cd(PBI) 2 in aqueous medium have been evaluated.