This paper will investigate the synergistic solvent extraction of extracted Pr from a chloride medium, using a mixture of two acidic extractants, namely 2-ethylhexylphosphonic mono-2-ethylhexyl ester (P507, HA) and di-2-ethylhexyl phosphoric acid (P204, HB) in kerosene. The stoichiometry of the extracted species was characterized by a classical lg-lg plot analysis. The composition of the extracted complexes has been determined as Pr(HA(2))(HB2)(2). Moreover, the differences of the extraction behavior from sulfuric and chloride medium were discussed in detail. The values of equilibrium constant and thermodynamic parameters such as Delta G, Delta H, and Delta S were also calculated. A cation-exchange mechanism was proposed and further clarified by slope analysis and IR spectra.