The samples of fraction nanostructured compositions containing magnetite (Fe3O4) and alpha-iron (alpha-Fe), obtained by welding of steel using a bare welding wire of Sv08G2S (ER70S-6) type in CO2 as a shielding gas, were characterized by X-ray phase and FT-IR spectroscopy, water vapor adsorption, and pH-metry and also were tested in the reaction of ozone decomposition at the initial ozone concentration, of 1.0mg/m(3). From the experimental data, it has been found that magnetite phase is responsible for the activity of the two-component, Fe3O4 + alpha-Fe, composition in the reaction of ozone decomposition.