Abstract. The processes of condensation of sulfur trioxide SO 3 and water vapor H 2 O. Sulfuric anhydride is formed in flues of thermal power plants (TPP) by partial oxidation of SO 2 (up to 5 % of the total SO 2 ) from the combustion of high-sulfur fuels, and belongs to a class of mild-hazard products. Sulfuric anhydride in the interaction with water vapor, which refers to greenhouse gases, under certain conditions, it forms sulfuric acid.