Heat transfer to boiling liquids is of primary industrial importance. Surprisingly enough organized study of the variables which affect heat transfer to boiling liquids has been far less than proportional to the industrial usage of this type of heat transmission. Perhaps the least investigated phenomenon involved in heat transfer to boiling liquids is the so~called critical state or maximum in the rate of heat transfer-thermal driving force relationship. This maximum or peak is believed to be caused by a change in the type or method of heat transfer from the heating surface to the boiling liquid. The system is said to pass from a state of nucleate boiling through the maximum to a state of film boiling. The critical point is known to very for various liquids yet no adequate correlations were found which would provide a prediction of the characteristics of heat transfer for binary mixtures of liquids. In as much es ethanol and benzene are used extensively in industry they were selected for use in this investigation. The purpose of this investigation was to determine the characteristics of heat transfer from a horizontal silver surface to boiling mixtures of ethanol and benzene. A horizontal plate evaporator, with the necessary accessory equipment for measurement and control, was designed and constructed. The test liquids were prepared for concentrations of ethanol in benzene from 0 to 100 per cent in 10 volume per cent increments. These test liquids were charged to the evaporator and the characteristics of heat transfer determined by a series of steady state conditions of heat transfer. The rate of heat transfer was evaluated from the wattage input to the electrical heating unit. The temperature gradient between the heating surface and the main body of liquids was determined by evaluating the readings of thermocouples placed in the liquid space and the heater plate. The maximum rate of heat transfer was considered equivalent to the heat flux which caused the boiling system to shift through the maximum in the heat flux-temperature gradient relationship. The critical temperature gradient was obtained by an extrapolation of the heat flux-temperature gradient curve to maximum heat flux. The general conditions for the tests were: heat transfer surface, silver; cold liquid height in evaporator, 4—1/2 inches; evaporator diameter, 1·31/32 inches. The tests were made at normal atmospheric pressure which varied from 710.0 to 718.0 millimeters of mercury pressure. Steady state conditions of heat transfer were established before data was taken.