A thermodynamic model is presented for the extraction of phosphoric acid from water by tri-n-butylphosphate (TBP), from aqueous solutions containing 0-6 moUkg phosphoric acid. The activity coefficient of phosphoric acid was calculated from experimental data using Pitzer's equation, and those of the components in the organic phase were calculated by Sergievskii-Danus's relationship. The proposed model gives a satisfactory description of the distribution of phosphoric acid and water, in the considered concentration range, by taking into account the formation of the following species: TBP, TBP.H2O, H3PO4.TBP and (TBP)(2).H3PO4.H2O.