The application of ultrasound is shown to be an excellent method to transform the dynamic instruments for the determination of vapour-liquid equilibrium (VLE) data into useful equipment for the determination of isobaric vapour-liquid-liquid equilibrium (VLLE) data. After a review of the few experimental isobaric VLLE data published in literature and the analysis of the reasons why conventional VLE instruments are not useful for systems of limited miscibility in the liquid phase, the positive effects of applying ultrasound are shown. The best place and position of the ultrasonic homogenizer in the instrument is discussed. Finally, experimental VLLE data obtained with the resultant instrument are presented for five ternary systems at 101.3 kPa: water+ethanol+ethyl acetate, water+ethanol+1-butanol, water+2-propanone+2-butanone, water+ethanol+diethylether and water+1-butanol+n-butylacetate.