Experimental investigations clearly show that the activity of a diesel oxidation catalyst (DOC, Pt-gamma-Al2O3) is changing during NO oxidation. This can be attributed to the reversible formation of platinum oxide. Therefore the activity depends strongly on the pretreatment and the operating conditions. In this paper, the impact of O-2 and NO2 as oxidising agents and of NO as reductant for platinum as well as the influence of different pretreatments is experimentally studied under typical DOC conditions in an isothermal flat bed reactor with either constant temperature or with temperature ramps. Furthermore, a method to determine the platinum oxide fraction in situ is shown. An inverse hysteresis in NO conversion is observed if the temperature is linearly increased and subsequently decreased at constant gas composition, confirming the findings of Hauptmann et al. [1]. A variation of the minimal temperature of the ramp shows that a reduction of PtO by NO is possible in a net oxidising atmosphere (500 ppm NO, 12% O-2, 10% H2O) below 220 degrees C. Above 300 degrees C, PtO becomes instable. Experiments with and without NO2 prove that small NO2 concentrations (<200 ppm) have no influence on the progress of platinum oxidation in the presence of 12% O-2. Platinum oxidation is mainly driven by the high O-2 concentration, which is confirmed by an observed deactivation of CO oxidation after reductive pretreatment. In a mixture of CO and NO, the light-off and light-out behaviour of both components is strongly modified by the other component. (C) 2012 Elsevier B.V. All rights reserved.