sec-Octylphenoxy acetic acid and 1,10-phenanthroline were employed for the extraction of bismuth from chloride medium, showing evident synergistic effects on bismuth extraction. Stoichiometry of the synergistic extraction was determined, giving a logarithm value of equilibrium constant of 6.75, which was much higher than those obtained by single sec-octylphenoxy acetic acid or 1,10-phenanthroline. Thermodynamic studies indicated that the synergistic extraction was an endothermic and spontaneous process. Moreover, antagonistic effects were found when the mixture was employed for lead extraction, demonstrating its potential to be applied to the separation of bismuth and lead at low mole fractions of sec-octylphenoxy acetic acid.