The influence of pre-dosed oxygen on NO-C2H4 interactions on the surface of stepped Pt(3 3 2) has been investigated using Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). The presence of oxygen significantly suppresses the adsorption of NO on the steps of Pt( 3 3 2), leading to a very specific adsorption state for NO molecules when oxygen-NO co-adlayers are annealed to 350 K (assigned as atop NO on step edges). An oxygen-exchange reaction also takes place between these two kinds of adsorbed molecules, but there appears to be no other chemical reaction, which can result in the formation of higher-valence NOx. C2H4 molecules which are post-dosed at 250 K to adlayers consisting of O-18 and NO do not have strong interactions with either the NO or the O-18 atoms. In particular, interactions which may result in the formation of new surface species that are intermediates for N-2 production appear to be absent. However, C2H4 is oxidized to (CO2)-O-18 by O-18 atoms at higher annealing temperature. This reaction scavenges surface O-18 atoms quickly, and the adsorption of NO molecules on step sites is therefore quickly restored. As a consequence, NO dissociation on steps proceeds very effectively, giving rise to N-2 desorption which closely resembles that following only NO exposure on a clean Pt( 3 3 2), both in peak intensity and desorption temperature. It is concluded that the presence of O-18(2) in the selective catalytic reduction (SCR) of NO with C2H4 on the surface of Pt(3 3 2) does not play a role of activating reactants. (C) 2008 Elsevier B.V. All rights reserved.