The transport mechanism of oxygen activated on platinum catalysts was investigated with a system of nanoparticle layers (sandwich structure). The results indicate that activated oxygen is transported via the gas phase and that the generally accepted surface transport, so-called spillover, is negligible. However, the lifetime of the activated oxygen is very limited. In experiments with spatially separated Pt- and C-layers the lifetime was estimated to be 65 ms. The reduction of activated oxygen presumably occurs via attachment to other gas molecules or to reactor walls. The identification of the dominant transport mechanism and the quantification of the catalytic effect could provide valuable guidelines for the design optimization of catalytic coated diesel particulate filters.