The vapor pressure and vapor composition of the methanol–cyclohexane system were investigated at temperatures ranging from 25 to 50°. The same properties of the binary systems: acetone–methanol and acetone–cyclohexane, as well as those of the ternary system: methanol–cyclohexane–acetone, were determined experimentally at 25°. The total pressures and compositions of the vapor for solutions lying close to the critical solution temperature of the binary system or close to the plait point of the ternary system are constant within the range of experimental accuracy. All these systems show azeotropic behavior. The methanol–cyclohexane system has an equimolar excess Gibbs free energy, GE, at 25° of 384.7 cal/mol, while the corresponding equimolar value of GE for the acetone–methanol system at 25° is 102.6 and for the acetone–cyclohexane system, at 25°, it is 274.5 cal/mol.