The liquid-phase composition resulting from the absorption of nitrogen oxides into water, sodium hydroxide solutions and especially aqueous and acid solutions containing hydrogen peroxide, was studied at 20 degrees C for various operating conditions. It is shown that for a given gaseous content, the difference in liquid-phase compositions is only due to the importance of the absorption of nitrous acid, as the absorption kinetics of NO2, N2O4 and N2O3 species are similar in aqueous solutions. The nitrous acid contribution equals zero for water and is maximal for NaOH solutions, leading to the smallest and greatest absorption efficiencies, respectively. The best selectivity for nitrite formation is reached in caustic solutions and is increased by a small NOx oxidation ratio. for the absorption of NOx into solutions containing hydrogen peroxide with increasing acidity, due to liquid-phase nitrous acid oxidation catalyzed by hydrogen ions, rising values of HNO2 contributions and of the disappearing rate of nitrous ions in solution were noted. A satisfactory agreement was observed between the nitrite-nitrate compositions determined experimentally by capillary electrophoresis and predicted by previously developed or slightly adapted models.