Distribution ratio models play an important role in simulation of any mass transfer operation. Mass balance being same on a stage, concentration of solutes calculated depends on the distribution ratio values. Specific to nuclear fuel reprocessing, models available for calculation of distribution ratios are mostly empirical. Robust models based on the thermodynamics of phase equilibrium are not available at present. A recursive thermodynamic distribution ratio model has been developed for U + HNO₃ + Pu system contacted with tri-n-butyl phosphate using predictive activity coefficient models. UNIversal QUasi Activity Coefficient (UNIQUAC) model is used for organic phase while Extended-UNIQUAC is employed for aqueous phase. Final equilibrium on a stage is assumed to be the result of a series of intermediate quasi-static states, where the phases in each intermediate state are in internal equilibrium. Gibbs free energy change of extraction (ΔG_i), excess Gibbs free energy (ΔG^E) of phases have been calculated which can be further utilized to study the phase stability of streams containing high plutonium content. Modelling of Gibbs free energy change of extraction for U, Pu, and HNO₃ has been carried out using experimental distribution ratios from literature and activity coefficients from present work. Levenber– Marquardt nonlinear regression technique has been used for fitting the ΔGi data obtained in this work to a nonlinear function of aqueous, organic phase concentrations and temperature. The developed thermodynamic distribution ratio model has been compared with distribution ratios obtained from correlations proposed by Horner, later modified by Watson and Rainey.