The effect of nitrogen dioxide on the physicochemical processes which take place during the absorption of sulfur dioxide in aqueous solution under conditions similar to those of a wet flue gas cleaning process has been studied. The study is done at 298.65, 318.45, and 333.35 K and ambient pressure. The gas phase and the liquid phase could be specified completely. The results show that after a few hours NO2 vanishes from the gas phase. This leads to a reproducible frozen (stable) state in which tetravalent sulfur exists in phase and chemical equilibrium. The simultaneous absorption of SO2 and NO2 in water depends on their initial concentrations and the temperature and is governed by the kinetics of a number of concurrent and consecutive reactions in the liquid phase. The gas phase contains SO2, N2O, and NO. The liquid phase contains a number of nitrogen-sulfur compounds besides tetravalent and hexavalent sulfur and nitrate. A scheme for the reactions that can take place in aqueous solution has been proposed. The analysis results together with the known thermodynamic and kinetic data from the Literature have been used to develop a model for the description of the SO2/NO2/N-2/H2O system. The model has been used to predict not only the stable state composition at various temperatures but also the time dependence of the system composition. The agreement between the experimental results and model calculations is quite good. The effects of temperature and the initial concentration of the gas mixture on the absorption behavior of SO2 are discussed.