This paper deals with the preparation and modification of iron oxide catalysts with different alkali metals. Among the prepared catalysts, Fe1.9K0.1O3 proved to be the most promising catalyst for the simultaneous removal of NOx and soot from diesel engines exhaust and was selected for the rest of investigations. The present study has shown that long-time treatment leads to a decline in the activity, and remains constant after at least 20 TPR experiments. This shows that the used catalysts still possess considerable activity. On the other hand long-time treatment causes a significant enhancement of N-2 selectivity, and the formation of by-product N2O was not observed. This alteration of catalytic performance is likely due to agglomeration of the promoter potassium being present at surface. Catalytic performance of the used Fe1.9K0.1O3 catalyst was also carried out in a more realistic diesel exhaust gas with two different types of feed gas compositions. This study confirms that Fe1.9K0.1O3 is a suitable catalyst for simultaneous removal of soot and NOx between 350 and 480degreesC. It is assumed that (CO) intermediates, formed by catalytic reaction of NOx and oxygen with soot surface, play an important role in NOx-soot conversion. (C) 2004 Elsevier B.V. All rights reserved.