CO oxidation with oxygen and ozone-oxygen mixtures was studied over a platinum/alumina monolith catalyst. Temperature ramp experiments were combined with mean-field Simulations to Study the reaction mechanisms. In the absence of ozone, a slow CO oxidation reaction was observed at low temperatures. The rate of this slow reaction was proportional to the square root of the oxygen pressure and independent of the CO concentration. At higher temperatures, the three-step Langmuir-Hinshelwood reaction mechanism dominated the CO oxidations. When some of the oxygen was exchanged for ozone, rapid oxidation of CO by ozone was observed. The suggested explanation was an Eley-Rideal mechanism, in which colliding ozone reacts with adsorbed CO oil the platinum surface. When this additional reaction step was included in the model, the Simulation results indicated a reduction in the bulk CO pressure. The experimentally observed ozone promotion of CO oxidation was thus attributed to a decrease in CO Surface self-poisoning. (c) 2006 Elsevier Inc. All rights reserved.