Surface species formed during co-adsorption of acetone and nitrogen oxides on Cu-ZSM-5 were studied by infrared spectroscopy to help address intermediacy during selective catalytic reduction of NO by hydrocarbons in excess oxygen (SGR-HC). NO plus acetone on Cu-ZSM-5 produced only marginal spectral differences compared to acetone on Cu-ZSM-5, indicating limited interaction between the adsorbates. Oxygen addition to NO plus acetone on Cu-ZSM-5 resulted in considerable NO2 formation. Upon oxygen addition, bands assigned to surface carbonyls and carboxylates formed from acetone attenuated substantially, apparently due to a strong affinity of NO2 for these ketone-derived surface oxygenates. Go-adsorbed NO2 and acetone on Cu-ZSM-5 resulted in a weak band at 2590 cm(-1). The co-adsorption data show that interaction of acetone was greater with the NO2 oxidant than with NO or oxygen. Addition of oxygen to NO plus acetone on Cu-ZSM-5 produced stronger interactions between oxidants and the organic reductant than those observed for the propene-containing system, especially at elevated temperatures (225 degrees C). The results suggest that both oxygenate and NO2 intermediacy may benefit SCR-HC.