(Liquid + liquid) equilibrium (LLE) data for the ternary system (water + butyric acid + isoamyl alcohol) have been determined experimentally at T = (298.15, 308.15 and 318.15) K. Complete phase diagrams were obtained by determining solubility and the tie-line data. Tie-line compositions were correlated by Othmer-Tobias method. The UNIFAC method was used to predict the phase equilibrium in the system using the interaction parameters determined from experimental data between groups CH3, CH2, CH, COOH, OH and H2O. It is found that UNIFAC group interaction parameters used for LLE could not provide a good prediction. Distribution coefficients and separation factors were evaluated for the immiscibility region. (c) 2004 Elsevier Ltd. All rights reserved.