The equilibrium data of quaternary system {phosphoric acid + hydrochloric acid + water + tri-n-butyl phosphate (TBP)} was determined at T = 303.2 K and atmospheric pressure by the method of components analysis. It was investigated experimentally over the mass concentration range (1 to 11)% hydrochloric acid and (9 to 21)% phosphoric acid in the initial aqueous phase. With increasing concentration of phosphoric acid, the distribution coefficients of phosphoric acid changed gently, and with the increased content of hydrochloric acid, the distribution coefficients of phosphoric acid increased, while the separation factor of hydrochloric acid decreased with the increase of phosphoric acid concentration. The experimental tie-line results were correlated with the non-random two-liquid (NRTL) activity coefficient model, and the reliability of the experimental values was determined through the Hand and Othmer-Tobias plots. The Othmer-Tobias and NRTL models are suitable for the system. (C) 2014 Elsevier Ltd. All rights reserved.