The catalytic oxidation of nitric oxide to nitrogen dioxide in the presence of sulphur dioxide over a standard Pt/SiO2 catalyst (EuroPt-1) was studied. The gas-phase reactions between nitric oxide, sulphur dioxide and oxygen were found to be insignificant under the experimental conditions concerned. The Pt/SiO2 catalyst was observed to be very active in catalyzing the reactions both of NO + O2 to NO2 and of SO2 + O2 to SO3. In the presence of sulphur dioxide, the catalytic activity for nitric oxide oxidation dropped dramatically, and in contrast, the presence of nitric oxide promoted sulphur dioxide oxidation. There is significant influence of NO/NO2 on sulphur dioxide oxidation; there are indications from temperature-programmed desorption and temperature-programmed reduction (TPR) that some sort of complex species (e.g. [Nx-Sy-Oz]) is formed on the platinum surface. Deactivation of the catalyst was also observed during the activity tests. It is believed that the deactivation of the catalyst was mainly caused by the build-up of some nitrogen and sulphur compounds on the active sites of the platinum catalyst. Another reason for the deactivation could be the sintering of platinum particles in the reaction environment. This has been confirmed by X-ray photoelectron spectroscopy, TPR and platinum dispersion measurements.