Vapor-liquid equilibria of a water+ 1-butanol system containing a trace amount of furfural were measured at atmospheric pressure by use of a lino-type still for systems of limited miscibility. Vapor-liquid compositions for the major components (water and I-butanol) are shown to be nearly coincident with those of the binary system. In the partially miscible region, the vapor-liquid equilibrium ratios of the trace component (furfural) at bubble point were found to be 2.5 and 0.46. Consequently, the partition coefficient of the trace component between the two liquid phases is 5.4. The equilibrium ratio curve of the trace component is presented, in which the calculated curve within the partially miscible region is shown to be coincident with the experimental data.