Catalytic properties of MnOx/Al2O3 catalysts were studied in the CO oxidation reaction. The catalysts were prepared by co-precipitation and then the product was calcined in Ar or air at temperatures of 700-1200 degrees C. It was found that increase in the calcination temperature to 950-1000 degrees C lead to the growth of catalytic activity for both series, especially for catalysts calcined in air. An increase in the catalytic activity of the catalysts calcined in Ar was related to the formation of Mn3-xAlxO4 cubic spinel which was stable during specimen cooling in inert atmosphere. It was found that active component in the air-calcined catalysts was formed via decomposition of the high-temperature precursor (cubic spine] Mn3-xAlxO4) followed by the appearance of aggregates consisting of imperfect Mn3O4+delta oxide and amorphous Mn-Al-O phase. The decomposition was accompanied by the formation of weakly bound oxygen which appears to be active in oxidation reactions. The structure of the active component was directly related to the composition of the high-temperature precursor - the higher the concentration of manganese cations are in the Mn3-xAlxO4 cubic spinel, the more Mn3O4 and weakly bound oxygen appear in the decomposition product. (C) 2013 Elsevier B.V. All rights reserved.