Four samples prepared by adsorption of heavy metal ions (Cu2+ Co2+ Mn2+ and Ni2+) on granulated blast furnace slag have been investigated as catalysts in oxidation reactions of CO and C6H6. Three different catalytic activity tests were used: oxidation with oxygen, oxidation with mixture of ozone with oxygen (OZCO method), and ozone decomposition. It have been found that the catalytic activity and the effectiveness of ozone application followed the sequence Cu/SL > Mn/SL > Co/SL > Ni/SL. The application of the ozonized oxygen leads to a strong decrease in the reaction temperature, whereupon the maximum effectiveness is being preserved. On the based of results obtained by different methods as mass titration, X-ray and EPR it was found that the active phases of these catalytic systems are compounds, containing catalytic metals dispersed in general by exchange on the surface of the blast furnace slag, used as a carrier. (C) 2004 Elsevier B.V. All rights reserved.