The influence of the reducing agent on the selective reduction of NOx over Ir black was investigated with special respect to the by-products formed. Propene, propane, CO, and H-2 were used as reducing agents in the presence and absence of oxygen and with either NO or NO2 as NOx component. Though all reducing compounds applied were able to reduce IrO2 in a similar way, it was found that only propene exhibits appreciable activity for the reduction of NO under oxidizing conditions. In oxygen-free atmospheres under large excess of the reducing agent, with propene considerable amounts of HCN were formed and with H-2 the major product was NH3. Furthermore the decomposition of propene, propane, butene, and 2-methyl-propene over Ir black and the activity of the produced carbonaceous deposits for NO reduction was investigated. In the sequence of propene > butene > 2-methyl-propene > propane the amount of carbonaceous deposits decreased in line with the ability of NO reduction. This corroborates the crucial role of carbonaceous deposits in the NO reduction process and supports an adsorbate-assisted mechanism of NO reduction. The presence of NOx shifts propene oxidation light-off to higher temperatures irrespective of the NO, component used. Experiments using NO2 as NOx component usually did not change the reaction pattern as compared to the situation with NO. This adds further evidence that NO2 is no necessary reaction intermediate in the reduction of NO over Ir black.