Vapor liquid equilibria (VLE) of the binary systems n-butanal + butyric acid and n-butanol + acetic acid were determined at two pressures, 26.65 and 53.33 kPa. The equipment used was a flow ebulliometer, which is ideal for reactive systems . The quality of the measured P-T-x-y data was verified by applying the thermodynamic consistency test of Van Ness and Fredenslund. The binary interaction parameters for the determination of the liquid-phase activity coefficients, represented by the models Wilson, UNIQUAC, and NRTL, were adjusted by using the maximum likelihood method. The nonideality of the vapor phase was considered by using a chemical theory with the correlation of Hayden and O'Connell for the calculation of the second virial coefficient and the prediction of the chemical equilibrium dimerization constant.