A systematic study of the solvent extraction of uranium(VI) and thorium(IV) from hydrobromic acid media was performed using the neutral phosphine oxide extractant Cyanex-923 in toluene. These metal ions were found to be quantitatively extracted with Cyanex-923 in toluene in the acidity range 5x10-5-1x10-4 M and 5x10-5-5x10-3 M, respectively, and they are stripped from the organic phase with 7.0 M HClO4 and 2.0- 4.0 M HCl, respectively. The effect of the equilibrium period, diluents, diverse ions and stripping agent on the extraction of U(VI) and Th(IV) was studied. The stoichiometry of the extracted species of these metal ions was determined based on the slope analysis method. The extraction reactions proceed by solvation and their probable extracted species found in the organic phase were UO2Br2 2Cyanex-923 and ThBr4 2Cyanex-923. Based on these results, a sequential procedure for their separation from each other was developed.