Depletion of nitrous acid from its aqueous solution is studied over a wide range of experimental conditions. The rate and stoichiometry of this process are determined as a function of mass transfer characteristics of gas-liquid contacting devices. It is found that the depletion of nitrous acid is brought about by three different mechanisms: 1) simultaneous hydration of N2O4 and evolution of NO, both existing in equilibrium with HNO2, 2) desorption of nitrous acid molecules and 3) decomposition of HNO2 into NO and NO2 occurring in the vicinity of the liquid surface. A quantitative analysis is made using the various rate and equilibrium constants with the oxides of nitrogen. The relative importance of the three mechanisms and, hence, the rate and stoichiometry of the whole process vary with the mass transfer characteristics of the gas-liquid contactors. The rate constants of the hydration of N2O4 and of the decomposition of nitrous acid (2HNO2→NO+NO2+H2O) as well as the solubility of N2O4 were established.