Tri-n-butylphosphate (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 16 molal 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 (H2O center dot TBP) over bar at lower water activities and the di-hydrate (H2O center dot TBP) over bar as activity further increased. HCl was extracted as the tri-solvate <(HCl(H2O)(7)center dot 3TBP)over bar> at low acid concentrations, with the di-solvate <(HCl(H2O)(6)center dot 2TBP)over bar>, and mono-solvate <(HCl(H2O)(3)center dot TBP)over bar> 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 <(HCl1.6(H2O)(1.8)center dot TBP)over bar>. The speciation also depends upon the extent of the dilution of TBP within the organic phase. Thus, tri-solvates <(HCl(H2O)(7)center dot 3TBP)over bar> are not present in undiluted TBP below 2 mol/L HCl, but are present in TBP diluted in aliphatic solvent. (C) 2012 Elsevier Ltd. All rights reserved.