Several mechanical mixtures of Ir black or IrO2 with various mixing materials like silica xerogel, alumina, and H-ZSM-5 were tested for their activity in the selective reduction of NO by propene. Independent of the mixing material used the unsupported Ir showed outstanding DeNOx activity compared to their supported counterparts. The conversion of NO to N-2 under lean conditions as well as oxidizability and reducibility of the Ir-IrO2 system was found to depend strongly on the Ir crystallite size. With increasing crystallite size, selectivity to N-2 is greatly enhanced mainly by the suppression of a reaction pathway leading to the production of NO2. Tailoring of Ir crystallite size by controlled sintering of Ir black afforded highly active Ir catalysts without the need for further conditioning on stream. Among unsupported platinum group metals iridium showed the best performance in NOx removal, The catalytic behaviour of Ir black was compared to the behaviour of Pt black, Pd black, and Rh black, Unsupported IrO2 with relatively large crystallite size (24 nm) was inactive for the selective reduction of NO with propene. Only IrO2 with small crystallite size (below ca. 11 nm) and high surface area could be activated by on-stream conditioning. Ir black and IrO2 were characterized by XRD, TEM, SEM, thermal analysis, and krypton adsorption measurements. It was shown that it is unlikely that NO2 is an intermediate in the reduction of NO.