The catalytic oxidation of methyl isobutyl ketone (MIBK) and o-xylene was studied on different catalysts. Oscillations in the molar flow rate of CO2 were observed in some systems, corresponding to apparent conversions into CO2 exceeding 100%. The oscillatory behaviour was found to be associated to the presence of strongly adsorbed compounds at the temperature of the reaction, which could be detected by TPO and TPD. The phenomenon is therefore determined by the strength of adsorption of the VOC in the catalyst support. (C) 2004 Elsevier Inc. All rights reserved.