Isobaric vapor-liquid equilibrium data for propionic acid + n-butyl propionate mixtures at 60, 80, and 101.3 kPa were provided. From the measured data, it can be seen that the relative volatility was close to unity and no azeotrope occurred. The experimental data could be correlated by the NRTL, Wilson, and UNIQUAC thermodynamic models used to calculate activity coefficients of the liquid and the Hayden-O'Connell model for the association of propionic acid in the vapor. Double azeotropes at the pressures below 17.4 kPa were predicted by the NRTL and Hayden-O'Connell model. Both the negative and positive azeotrope temperatures were found to be close to those reported in the literature. The UNIFAC model with no adjustable parameters was also used in this study and was found to adequately describe the VLE behavior when the liquid-phase composition of propionic acid was below 0.85.