Vapor-liquid equilibria of two binary systems, benzene-n-hexane and benzene-cyclohexane, were measured at pressures from 4 to 18 atm. The three-constant RedlichKister equation was found satisfactory for correlation.With the benzene-n-hexane system no azeotrope occurred ; with the benzene-cyclohexane system an azeotrope appeared, and the mole fraction of cyclohexane in it diminished with increase in pressure. Lack of correction for fugacity in the vapor phase introduced a maximum error of 4'% in activity coefficients at highest pressure.Previously published data on these two normal boiling points of the azeotrope and systems have been restricted to measuren-hexane. ments a t atmospheric pressure and haveVapor-liquid equilibria and thermodyconflicted as to the occurrence or absence of namic properties of the benzene-cyclohexane an azeotrope in the benzene+-hexane sys-system have been established by Scatchard, tem. Tonoberg and Johnston (11) reported Wood, and Mochel (10) and by Wood and vapor-liquid-equilibria data on the benAustin (12), who attribute the highly zene-n-hexane system a t atmospheric presirregular behavior of this system to the sure and found no azeotrope, Other investigators ( I T , 5, ' 7, 6) reported the formation of an azeotrope. Griswold and Ludwig (b), in commenting on the data of Tonoberg and Johnston. stated that the azeotrope perhaps escaped detection because of the small temperature difference, 0.1 "C., between the large entropy of mixing. Richards and Hargreaves (9) established similar data on this system a t atmospheric pressure and found the azeotrope composition to be 50.2 mole % benzene a t a normal boiling point of 77.4"C. As the boiling points of the pure components are close together, a slight deviation from ideality readily produces a minimum boiling azeotrope (8). APPARATUS AND EXPERIMENTAL PROCEDUREThe apparatus used for measuring vaporliquid equilibria was a continuous liquidrecirculating still similar to that used by Griswold et al. ( I ) , as shown in Figure 1.It consisted of a still, condenser C, reboiler leg B, condensate reservoir CR, and calibrated spring-loaded nonreturn valve N (Figure 2).Temperature measurements were made with an iron-constantan thermocouple with its cold junction a t the melting point of iceTemperatures recorded by the thermocouples, being slightly lower than the true values owing to heat losses along the thermocouple wires, were made by use of the following equation for the size of wire used: to = 0.000 15686 t" + 0.9996 t + 0.856 (1) mhere to = true trmpernture and t = observed ternpcrature. Pressures were measured by a Bourdon gauge having a scale range of 0 to 600 lb./sq. in. and a dial 8 in. in diameter with divisions mnrked a t 2 lb./sq. in. intervals. This gauge was calibrated periodically against a dead-weight gauge. A Doran potentiometer in conjunction with a multifluu-spot galvanometer was used to record potentials (Figure 3). Vaporpressure-temperature relationships were measured for three pure compounds, benzene (b.p. 80.1°C.), cyclohexane...