Isothermal vapor-liquid equilibrium (VLE) data were measured for propionic acid + butyl propionate at 373.15 and 393.15 K, and isothermal vapor-liquid-liquid equilibrium (VLLE) data were also measured for n-butanol + water, butyl propionate + water, and water + n-butanol + butyl propionate at temperatures ranging from 323.15 to 393.15 K. No azeotrope was found in propionic acid + butyl propionate. The mutual solubility data of the binary aqueous systems were correlated well with the NRTL model accompanying with temperature-dependent parameters. Improvement on the calculation of saturated vapor compositions has been made by using two-term virial equation with one adjustable binary interaction parameter to represent the non-ideality of the vapor phase. The model parameters determined from the binary VLLE data of n-butanol + water and butyl propionate + water and the binary VLE data of n-butanol + butyl propionate are capable of predicting satisfactorily the VLLE properties for the ternary system of water + n-butanol + butyl propionate. (C) 2003 Elsevier B.V. All rights reserved.