The effects of CO2 and H2O on the NOx storage and reduction characteristics of a Pt/Ba/Al2O3 catalyst were investigated. The presence of CO2 and H2O, individually or together, affect the performance and therefore the chemistry that occurs at the catalyst surface. The effects of CO2 were observed in both the trapping and reduction phases of the experiments, whereas the effect of H2O seems limited to the trapping phase. The data also indicate that multiple types of sorption sites (or mechanisms for sorption) exist on the catalyst. One mechanism is characterized by a rapid and complete uptake of NOx. A second mechanism is characterized by a slower rate of NOx uptake, but this mechanism is active for a longer time period. As the temperature is increased, the effect of H2O decreases compared to that of CO2. At the highest temperatures examined, the elimination of H2O when CO2 is present did not affect the performance.