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Abstract: Tri-n-butyl phosphate (TBP) is a common solvent used for extracting metal ions from acidic solutions. In this work, the extraction of water and acid by TBP was modelled up to 16molal HCl, which is considerably above the range of existing work. The solubility parameter and thermodynamic equilibrium constant for the formation of each extracted species were determined using both experimental and literature data. The resulting model was able to successfully predict both the water and hydrochloric acid extraction by undiluted TBP or TBP diluted in aliphatic solvent within this concentration range. Water was shown to be extracted as the mono-hydrate at lower water activities and the di-hydrate as activity further increased. HCl was extracted as the tri-solvate at low acid concentrations, with the di-solvate , and mono-solvate forming as acid concentration increased. The present results show that at high acid concentrations, water is rejected from the organic phase as the acid is extracted, with the species formed having a typical composition of . The speciation also depends upon the extent of the dilution of TBP within the organic phase. Thus, tri-solvates are not present in undiluted TBP below 2mol/L HCl, but are present in TBP diluted in aliphatic solvent. [Copyright &y& Elsevier]