The distribution equilibrium of vanadium (IV) between solution of bis(2,4,4-tri-methylpentyl)phosphinic acid (BTMPPA, HR) dissolved in EXXSOL D80 and weakly acidic sulfate solution was investigated as functions of the concentration of extractant, vanadyl ion, hydrogen ion and sulfate ion in the aqueous phase. The stoichiometry and the chemical structure of the extracted species of vanadium (IV) were determined on the basis of slope analysis and IR spectra. The extraction reaction proceeds according to a cation exchange mechanism, in which vanadyl ion VO2+ replaces hydrogen ions of P-O-H and coordinates with oxygens in P=O in the BTMPPA and forms the complex VOR(2) . 2HR with a possible structure containing eight-membered chelating rings. Temperature dependence of the extraction equilibrium constants was examined to estimate the apparent thermodynamic functions of extraction reaction (Delta H, Delta S and Delta G).