‫اﻟﺜﻮرﻳﻮم‬ ‫اﺳﺘﺨﻼص‬5-n 4 n − ABSTRACT: Solvent extraction of Thorium (VI) ion from perchlorate solution using didodecylphosphoric acid, DDPA, in chloroform diluent was studied. The effects of stripping hydrochloric acid concentration, stripping time, extraction time, DDPA concentration, pH, ionic strength, supporting electrolyte and temperature on the extraction processes have been studied. From the distribution coefficient values at different temperatures, the enthalpy, the free energy and the entropy changes associated with the extraction processes were determined. The composition of the complex formed was established to be Th(ClO 4 ) 4-n R n (HR) 5-n where, n=1 or 2 and (HR) 2 represents the dimer of DDPA.