Abstract: Equilibrium properities of the partially miscible system of water and isobutyl aldehyde were difficult to measure due to the latters chemical activity, and as such, the accuracy of literature data is generally poor. Effective steps were taken in the present work to prevent oxidation and polymerization of isobutyl aldehyde, thereby insuring good reproducibility of the data obtained. In this work, vapor-liquid equilibrium and liquid-liquid equilibrium data for this binary system, measured at 760 mm Hg, are correlated by using both the NRTL equation and the multiparameter Wilson equation (Mitsuyasu method). Three sets of binary parameters for each equation are determined in the following manner: (1)based on VLE experimental data alone, (2)based on LLE experimental data alone and(3) based on both VLE and LLE data. The vapor compositions calculated from these two equations with the three sets of binary parameters all agree well with the experimental data.