N235 in isoamyl alcohol solution acts as an efficient extraction system for the revovery of hydrochloric acid (HCl) at very low concentrations in a coupled reactive extraction-crystallization process. In this study, the extraction equilibrium of N235 + isoamyl alcohol + HCl + H2O system was investigated systematically. The formation of the extraction complex in the organic phase was determined by using the experimental data and it was confirmed to be (HCl)(4)(R3N)(4)(H2O)(8). Further, the thermodynamic model was then established by using Pitzer equation to calculate the activities of all the species. The values of the interaction parameters obtained by regression are as follows: beta((org))(R3N,4:4:8) = 2.8782 and beta((org))(4:4:8,4:4:8) = 11.4764. The extraction equilibrium constant is ln K-4:4:8 = 38.0274. The thermodynamic model was demonstrated to be reliable and suitable for the N235 + isoamyl alcohol + HCl + H2O system.