A thermodynamic model that predicts the extraction of trivalent lanthanide (Pr, Nd) nitrates and water from aqueous solutions by tri-n-butylphosphate (30, 50 and 100 vol%) in dodecane is presented. The Mikulin-Sergievskii-Dannus’ model was used to predict the extraction isotherms of lanthanides (Ln) and water for Ln(N03)-3H30-TBP-dodecane system and Ln(N03)3-NH4N03-H20-TBP-dodecane system. The parameters of the model were used in conjunction with those previously published for nitric acid extraction by TBP, to predict the extraction isotherms of lanthanides and nitric acid for Ln(N03)3-HN03-H20-TBP 100 vol%system The stoichiometric mean activity coefficients of components in the aqueous phase were calculated from experimental data using Mikulin's equation, and those of the components in the organic phase were calculated by Sergievskii-Dannus'equation. A satisfactory description of the distribution of lanthanide nitrate and water is obtained over a wide range of lanthanide concentration in the aqueous phase by taking into account the formation of the following complexes: TBP,TBP H20,(TBP)3Nd(N03)3,(TBP)4Pr(N03)3, (TBP)3-Pr(N03)v It was shown that in Ln(N03)3-HNOrH20-TBP 100 vol% system, it is necessary to consider anacid-complex,(TBP)3-HLn(NO3)4, in order to obtain the best fit of the isotherms to the extraction data.