Flow reactor experiments and XPS measurements were used to investigate the observed decrease in activity of Pt/Al2O3 and Pt/BaO/Al2O3 catalysts during reaction conditions. The reactions studied were NO(g) + 1/2O(2(g)) double left right arrow NO2(g). The influence of platinum dispersion on activity and on the activity decrease with time was also studied. When the Pt/Al2O3 catalyst was exposed to NO2 (350degreesC), the NO2 dissociation rate decreased. This activity decrease with time was also studied with XPS and it is suggested that it was due to platinum oxide formation. Further, when the platinum dispersion was increased, the platinum activity decreased and the reason may be that small platinum particles more easily form oxides. According to XPS measurements platinum supported on BaO/Al2O3 gave a higher concentration of platinum oxide after NO2 exposure compared to what was found for Pt supported on only Al2O3. Flow reactor experiments also showed that Pt/BaO/Al2O3 was less active, both for oxidation of NO and for reduction of NO2, than Pt/Al2O3 was and one possible explanation for this is the fact that platinum on BaO/Al2O3 forms more oxides.