Abstract: A method and apparatus for determining the phase behavior of gas/oil mixtures at preselected controlled high temperatures and high pressures wherein a stream of oil and gas under controlled conditions of volume rate and pressure are separately introduced into a mixing chamber and then into a separator wherein the liquid oil phase separates by gravity into the bottom of the separator and the gas phase migrates by gravity to the top of the separator. The gas phase of the separator is in fluid communication with a pressure control system. Samples of oil and gas are separately withdrawn from the separator after thermo-dynamic equilibrium is reached in the mixing chamber and each sample is analyzed to measure the gas/oil ratio.