Abstract: A simple and rapid method has been proposed for the determination of the pK2aE, pKaE and K2 of Di-2-ethyl-hexylphosphoric acid between water and benzene by the two phase titration method. The principle of this method is similar to that of the pH method used in coordination chemistry. The special feature in our treatment is that we will consider the entire equilibrium existing in the two phases.The relation between Ka, Kd and KaE is expressed byWhen HDEHP is soluble in benzene, the main form is a dimer of the formula (H2A2), therefore the form HA existing in aqueous phase can be neglected. The data obtained by the method of the graphic curvefitting agree with those obtained by means of mathematical analysis. From this it is clear that the assumption used is correct. The results are as follows: log K2 = 3.85, pKaE = 6.0 and pK2aE = 15.8 (25±0.2℃).