The effect of sodium promotion on the performance of an iridium black catalyst for the selective catalytic reduction of NO by propene has been investigated. Sodium loadings above 1 at% were found to increase the nitrogen selectivity close to 100% in the whole temperature range investigated (150-450 degreesC). However, the presence of Na shifted the on-set of all reactions to higher temperatures (ca. 20-50 degreesC for the highest promoter loadings, i.e., 10 at% Na), leading to a narrowing of the operation temperature window. Consequently, moderate sodium loadings in the range of 1-3 at% represent the best compromise for a beneficial application of iridium in DeNO(x) catalysis. Combined thermogravimetry-mass spectrometry investigations revealed that not only NO dissociation, but also, e.g., the decomposition of propene and the adsorption of oxygen as well as the oxidation of iridium are influenced by the presence of sodium. Mechanistically, our observations are consistent with a model, where the removal of adsorbed oxygen could represent the rate-limiting step under the applied conditions.