INTRODUCTION Over the past several years the American Petroleum Institute has sponsored research to obtain liquid-liquid-vapor equilibrium data for many hydrocarbon-water systems. In an effort to more completely characterize the phase behavior of these systems the present study was conducted to measure three-phase critical end points. The three-phase critical end point is the highest temperature at which three phases (water-rich liquid@ hydrocarbon-rich liquid@ and vapor phases) coexist. Above this temperature either the water-rich or the hydrocarbon-rich liquid phase disappears. To illustrate the behavior of water-hydrocarbon mixtures the P-T diagram for benzene/water is given in Figure 1 (Tsonopoulos and Wilson@ 1983). The vapor-pressure curves of the two pure components@ represented by light solid lines@ end at the respective critical points ( 562.09 K and 4.898 MPa for benzene and 647.30 K and 22.12 MPa for water). The heavy solid line represents the gas-liquid critical curve and is broken into two segments. The lower segment begins at the critical point of pure benzene and continues to the three-phase critical end point. The upper segment begins at the critical point of water and initially proceeds as expected toward the benzene critical point@ but at about 579 K reaches a minimum in pressure and turns upward@ reaches a minimum in temperature at about 569 K and 23.5 MPa (called the critical solution point) and increases in pressure and temperature to above the critical temperature of pure water. The broken line is the three-phase equilibrium curve which ends at the three-phase critical end point. In the region below the three-phase curve the two phases in equilibrium are a liquid phase and a vapor phase. Above the curve the two equilibrium phases are a hydrocarbon-rich liquid and a water-rich liquid. Above the three-phase critical end point only the water-rich liquid phase and the vapor phase exist. The three-phase equilibrium behavior of many waterhydrocarbon systems has been previously studied (Brady et al.@ 19821 and references cited therein) at conditions below the three-phase critical end point. The results of the present work complement these earlier results and help characterize the equilibrium properties of water-hydrocarbon systems.