The interaction of nitric acid in two—phase Bystems in the pH range with the extractant, N,N—N’,N’-octabutylimidodipho8photetramide (OBIPA) in toluene has been investigated using two—phase potentiometiic titrations. The data thus obtained were analysed by the ESTA library of programs which was designed for modelling solution equilibria in aqueous phases. The use of such a program in the analysis of two-phase potentiometric data is novel, and the feasibility and corrections needed to be applied are discussed. Gratifying results were obtained indicating that a program such as ESTA may indeed be used for two-phase systems. The extraction equilibrium constant for the formation of the (2HA.HNO3)ospecies was found to be 7.54 * 104. Constants could also be calculated for the dimerisation and the partition of the extractant between the two-phases. These were found to be 18.5 M1−and 1.92 * 102respectively. A water soluble, methyl analogue of OBIPA was used to calculate the acid dissociation constant (pKa) which wasfound to be 9.043.