The N 2O 4 - NO 2 gas system is convenient for experimental studies at relatively low temperatures of heat transfer processes in equilibrium-type reacting systems. The thermal conductivity and specific heat of these systems are unusually large and are non-uniform functions of temperature making difficult the assignment in heat transfer correlations of effective film values of these thermal properties. Rates of forced-convective heat transfer from a cylinder to N 2O 4-NO 2 gas at 1 atm were experimentally determined at free stream velocities up to about 40 ft/sec. If specific enthalpy instead of temperature is used as the driving force for heat transport a consistent correlation of the over-all heat-transfer rate data can be obtained by dimensional analysis, employing equilibrium properties in the conventional relationship, N Nu = φ ( N Pr, N Re) The advantages of using specific enthalpy as a driving force are justified from theoretical considerations. Due primarily to the high thermal conductivity of this system, values of conventional heat transfer coefficients based on a temperature driving force may be as high as eight times those for a similar non-reacting system.