Using the distribution data for di(2-ethylhexyl)phosphoric acid (D2EHPA or RH) between kerosene and 0.10 kmol m(-3) (Na+,H+)Cl-, the equilibrium constants of D2EHPA such as the dimerization, the distribution, and the acid dissociation constants were determined as K-2 = 3.39 x 10(4) m(3) kmol(-1), K-d = 1.66 x 10(3) kmol m(-3), and K-a = 1.99 x 10(-2) kmol m(-3), respectively. The extraction equilibrium formulation was established for the distribution of Mn(II) between kerosene containing D2EHPA and 0.10 kmol m(-3) (Na+,H+)Cl-. The composition of the extracted species is MnR2.(RH)(2) in the organic phase. The single species exists in the range of less than 20% of the extractant being converted to the complex. The extraction equilibrium constant of this reaction was also found at 30 +/- 0.5 degrees C.