Three-liquid-phase extraction has been considered to be a promising method for isolation and separation of multicomponents. Selective extraction-separation of Ti(IV), Fe(III), and Mg(II) in the three-liquid-phase system containing trialkylphosphine oxide (TRPO), poly(ethylene glycol) with an average molecular mass of 2000 (PEG 2000), and (NH4)(2)SO4 was achieved by adding a water-soluble complexing agent, ethylenediaminetetraacetic acid (EDTA). The simple and environmentally benign complexing method was proved to be an effective strategy for enhancing the selectivity of the PEG-2000-rich middle phase for Fe(III) without reducing the affinity of the TRPO-rich top phase to Ti(IV). The related chemistry was detailed, and effects of some important parameters such as the aqueous phase pH, EDTA amount, mixing time, and temperature were examined. It revealed that Ti(IV) and Fe(III) could be enriched respectively into the TRPO-rich top phase and the PEG-2000-rich middle phase through optimization of the aqueous pH and the EDTA amount. Ti(IV) was extracted into the top phase with a slow kinetics and a positive enthalpy change (Delta H), whereas Fe(III) was rapidly transferred to the middle phase as Fe(III) EDTA complexes formed by an exothermic reaction. Mg(II) tended to remain in the bottom phase.