Aqueous solutions of alkanolamines are often used to remove the acid gases (CO2 and H2S) from hydrocarbon streams. The application of the absorption process to liquid hydrocarbons is not as well developed as its use for gaseous hydrocarbon streams. As well, experimental data for systems of amine solutions and liquefied petroleum gases are scarce. New data have been obtained for the equilibria in the system n-butane-water-methyldiethanolamine (MDEA). A knowledge of the phase behavior in this system is needed to model the equilibria encountered in the quaternary and quinary systems of industrial importance. In this work vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibria have been measured at temperatures between 25 degrees and 150 degrees C at pressures up to 20 MPa for a 3 M solution of MDEA.