Iron oxide modified by potassium, i.e. Fe1.9K0.1O3, exhibits high catalytic performance for the simultaneous conversion of soot and NOx into CO2 and N-2. The present study shows that long-time treatment of the catalyst leads to a drastic decrease in the activity, whereas even the aged catalyst maintains considerable activity. On the other hand, long-time treatment causes selective N-2 formation, i.e. no more formation of the byproduct N2O. This alteration of catalytic performance is likely due to agglomeration of the promoter potassium being present at the surface of catalyst. Detailed experiments were carried out with a more realistic diesel model exhaust gas to confirm that Fe1.9K0.1O3 is a suitable catalyst for the simultaneous removal of soot and NOx between 350 and 480 degreesC. It was assumed that (CO) intermediates, formed by the catalytic reaction of NOx and oxygen with the soot surface, are the reactive species in NOx-soot conversion.