The selective catalytic reduction of nitric oxide with ammonia was studied using Fe2O3, Cr2O3 and CuO loaded active carbons as catalysts in the presence and absence of oxygen at reaction temperatures between 100 and 500 degrees C. Active carbons pretreated with concentrated nitric acid showed a higher catalytic activity in SCR than catalysts which were oxidized in air. The ash content (from 0.2 to 7.1 wt.%) of different unloaded active carbons had no effect on the catalytic activity below 300 degrees C in the absence of oxygen. However, in the presence of oxygen an increasing ash content resulted in an increase in activity. Transition-metal oxide loading led to an increase in SCR activity, especially in the absence of oxygen. An increasing transition-metal content from 1 to 10 wt.% improved the activity as well. The presence of oxygen in the reaction mixture enhanced the conversion of nitric oxide especially in the low-temperature range between 100 and 200 degrees C, Activity and selectivity of the respective catalysts were influenced by the type of metal oxide : in the presence of oxygen, catalysts with 10 wt.% Fe were the most active and selective.