The thermodynamic equilibrium constant for the vapor‐phase reaction NO+NO2+H2O⇌2HNO2 has been determined at reduced pressures by evaluating the equilibrium distribution of known quantities of the reactant gases by measurements of light absorption by nitrogen dioxide. The value of the equilibrium constant at 25°C is K1=2.38 atm—1, which corresponds to ΔF°=—0.512 kcal/mole. The free energy of formation of HNO2 vapor at 25°C and at isomeric equilibrium is ΔFf°=—11.019 kcal/mole, based on the equilibrium measurements and the free energies of formation of NO, NO2, and H2O.

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