The reduction of NO, in the presence of i-C4H10 or i-C4H8 and O-2, catalyzed by a platinum foil, was studied in order to understand better the catalytic activity of platinum metal, free of any support or dispersion effects. The reaction products were analyzed by mass spectrometry, and the surface was characterized by X-ray photoelectron spectroscopy at different stages of the reaction. A correlation between the catalytic activity for NO conversion and the presence of adsorbed intermediates is demonstrated. The role of oxygen is interpreted as twofold : formation of active intermediates and deactivation of the surface. The effect of traces of sulfur dioxide in the reacting phase on the reduction of NO by isobutene was investigated, The influence of SO2 is very much dependent upon its initial concentration in the gas phase. Low concentrations(<15 ppm) promote the reduction of NO, whereas higher levels poison the reaction by a surface site blocking effect,