In the field of nuclear toxicology, the knowledge of the interaction of actinides (An) with biomolecules is of prime concern in order to elucidate their toxicity mechanism and to further develop selective decorporating agents. In this work, we demonstrated the great potential of hydrophilic interaction liquid chromatography (HILIC) to separate polar thorium (Th) biomimetic peptide complexes, as a key starting point to tackle these challenges. Th 4+ was used as plutonium (Pu 4+ ) analogue and pS16 and pS1368 as synthetic di-and tetraphosphorylated peptides capable of mimicking the interaction sites of these An in osteopontin (OPN), a hyperphosphorylated protein. The objective was to determine the relative affinity of pS16 and pS1368 towards Th 4+ , and to evaluate the pS1368 selectivity when Th 4+ was in competition complexation reaction with UO 2 2+ at physiological pH. To meet these aims, HILIC was simultaneously coupled to electrospray ionization mass spectrometry (ESI-MS) and inductively coupled plasma mass spectrometry (ICP-MS), which allowed to identify online the molecular structure of the separated complexes and quantify them, in a single step. Dedicated HILIC conditions were firstly set up to separate the new dimeric Th 2 (peptide) 2 complexes with good separation resolution (peptide = pS16 or pS1368). By adding pS16 and pS1368 in different proportions relatively to Th 4+ , we found that lower or equal proportions of pS16 with respect to pS1368 were not sufficient to displace pS1368 from Th 2 pS1368 2 and pS16 proportion higher than pS1368 led to the formation of a predominant ternary complex Th 2 (pS16)(pS1368), demonstrating preferential Th 4+ binding to the tetra-phosphorylated peptide. Finally, online identification and quantification of the formed complexes when Th 4+ and UO 2 2+ were mixed in equimolar ratio relatively to pS1368 showed that in spite of pS1368 has been specifically designed to coordinate UO 2 2+ , pS1368 is also Th 4+ -selective and exhibits stronger affinity for this latter than for UO 2 2+ . Hence, the results gathered through this approach highlight the impact of Th 4+ coordination chemistry on its interaction with pS1368 and more widely to its affinity for biomolecules.