The extraction mechanism of Po(IV) from hydrochloric acid (HCl) solution by tributylphosphate (TBP) and trioctylamine (TOA) was investigated as a function of acid (0.1–12 mol/kg) and extracting agent (0.36–3.64 M) concentrations. Experimental data were modeled to determine the extraction mechanisms as well as the associated thermodynamic constants. Preliminary extraction experiments were carried out using tri-n-octylamine (TOA) in order to determine the activity-corrected Ringbom coefficient in concentrated HCl. The results showed the existence of a species involving two TOA molecules, R 3 NH ð Þ 2 PoCl 6 , which dominated organic polonium speciation in concentrated HCl (9–12 mol/kg). Then, we determined the complexation coefficient of PoCl 2À 6 as a function of HCl concentration in order to calculate PoCl 2À 6 molality for a known total concentration of polo-nium. The obtained information was used to determine the extraction mechanism occurring in the presence of TBP. The theoretical model agrees with the experimental results considering the existence of the following extracted polonium species: TBP ð Þ 2 Po OH ð Þ 2 Cl 2 and TBP ð Þ 2 HPoCl 5 in the acidic ranges of 0.1–1 and 2–9 mol/kg, respectively. Another extracted species seems to exist in intermediate HCl molality. However, the number of TBP molecules and the associated thermodynamic extraction constant could not be determined. Taken together, the data enable us to propose that the extraction scheme of polonium by TBP corresponds to the following two reactions: 2TBPþPoCl 2À 6 þ2H 2 O Ð ðTBPÞ 2 PoCl 2 ðOHÞ 2 þ2H þ þ4Cl À 2TBPþPoCl 2À 6 þH þ Ð TBP ð Þ 2 HPoCl 5 þCl À