Many catalysts have been tested for direct NO catalytic decomposition up to now, however, none has been sufficiently stable, active and selective at economically feasible temperatures [1]. It was found out that the catalytically less active oxides can be activated by the alkali promoters, further increase in the catalyst activity would be difficult to achieve with metals or alloys alone or simple metal oxides, a more active system might require a bifunctional surface presented e.g. on mixed oxides. Co-Mn-Al mixed oxide catalysts are active in N2O decomposition and VOC oxidation . In both cases, the existence of a cooperative effect between Co and Mn was published. Since potassium promoted Co-Mn-Al mixed oxides are also active for NO decomposition , the effect of Co and Mn substitution in K/Co-Mn-Al mixed oxide catalysts for direct NO decomposition was tested in this paper.