BACKGROUND: A three-liquid-phase system (TLPS) composed of an organic solvent-rich top phase, a polymer-rich middle phase and a salt-rich bottom phase is a newly emerging separation medium. Due to low affinity of the polymer-rich phase for metals it is necessary and important to search for a suitable complexing agent that has a definite affinity for the polymer-rich middle phase and a high selectivity for the metal ion of interest. RESULTS: Addition of 1,10-phenanthroline (phen) is effective in enhancing the separation of titanium and iron from magnesium in the TLPS consisting of trialkylphosphine oxide (TRPO)-PEG 2000-(NH4)2SO4. Hydrogen-bonding interactions between PEG 2000 and phen molecules are the driving force for anchoring tri(phen)-iron(II) sulfate complexes in the middle phase. Under the conditions (pH = 1.5, molar ratio of phen to iron(II) = 3.4:1), nearly 86% of titanium was extracted into the top phase while 100% of iron(II) was distributed in the middle phase, without any interference between the metal species. The separation factor of titanium and iron in the upper two phases was greater than 20 000. CONCLUSIONS: A single step of extraction and separation of titanium and iron from magnesium was realized in the TRPO-PEG 2000-(NH4)2SO4 TLPS with phen as additive. It highlights the effectiveness of TLPS in dealing with multi-metal solutions and suggests a potential use of TLPS in the separation of iron and other target metals. As iron is ubiquitous the separation of iron is often needed in both analytical processes and the hydrometallurgical industry. Copyright (c) 2012 Society of Chemical Industry