The determination of whether a phase is a:liquid or a vapor is somewhat arbitrary, particularly when the fluid is in the single-phase region. However, the calculation of multiphase flow in oil industry simulators is dependent upon the labeling of the phases. Key properties in flow modeling, such as relative permeabilities and capillary pressures, are dependent upon which phases are present. Correct labeling of the fluid phases is therefore critical to the success of simulations. Many methods have been proposed for identifying phases. These are reviewed here. Five methods have been selected for detailed comparison, including a new method, based on the thermal expansion coefficient. The results of the comparison and the relative advantages of the methods are discussed.