The reduction of nitrogen oxides by propene in the presence of air under net oxidising conditions has been studied for two Cu/alumina catalysts of low (1%) and high (5%) copper loadings in a flow microreactor and by DRIFT. The reaction was studied in the range 473-773 K using mixtures of 2.5% NO, 1% C3H6 and 10% O-2 with a balance of nitrogen or helium, using samples which were pretreated in air at 673 K and also over samples which had been pre-exposed to SO2 at 473 K. The surface species present under reaction conditions have been identified and the sensitivity of their adsorption sites in the two different loaded catalysts to SO2 pre-treatment has been investigated. SO2 adsorption enhanced NO adsorption at 473 K in the absence of oxygen and, in reaction, enhanced formation of NCO species leading to increased levels of adsorbed CO as a decomposition product.