The deactivation of NO oxidation by SO2 was studied with the use of commercial diesel oxidation catalysts (DOC) and Pt/Al2O3 as reference material, coated on cordierite monolith. Despite their slightly different elemental compositions, the NO conversion rates of the fresh commercial catalysts were very similar. The maximum NO conversion was 38% at 350 degrees C, and above this temperature conversion started to be limited by the thermodynamics of the reaction. The rates of NO conversion strongly decreased with the start of SO2 closing. For analysis Of SO2 conversion and uptake, SO2 and SO3/H2SO4 were determined separately in the gas phase by absorption and titration. Under typical exhaust gas conditions (1 ppm SO2, 250 degrees C, the catalysts functioned as sulfur traps and stored a large part of the emitted SO2. The SOx storage was divided into two phases: a fast saturation of the catalyst surface with sulfuric acid, which hampered NO conversion, and a slow, long-lasting sulfation of the washcoat. The storage capacities of the oxidation catalysts reached their rnaxima at 250 degrees C due to the temperature dependency of sulfur adsorption and desorption. Adsorbed sulfuric acid desorbed between 350 and 400 degrees C, whereas more stable compounds, such as aluminum sulfate, were decomposed at higher temperatures. Deactivated catalysts could be completely regenerated within a few minutes at temperatures above 350 degrees C. However, repeated or lengthier thermal treatments resulted in a reduced sulfur storage capacity and irreversible activity losses for NO oxidation due to a reduction of the active surface by sintering.