The effects of using NO or NO2 as the NO (X) source on the performance of a NO (X) storage/reduction catalyst were investigated from 200 to 500 A degrees C. The evaluation included comparison with constant cycling times and trapping the same amount of NO (X) during the lean phase. With NO2 as the NO (X) source, better trapping and reduction performance was attained in comparison to NO, at all operating temperatures except 300 A degrees C. This exception, under the conditions tested, was likely due to high NO oxidation activity and rapid trapping of NO2, although it is expected that extending the trapping time would lead to consistent differences. Several reasons for the observed improvements at 200, 400 and 500 A degrees C with NO2 relative to NO are discussed. One that can explain the data, for both trapping and release improvement, is treating the monolith as an integral reactor. With NO2, more NO (X) is trapped at the very inlet of the catalyst, whereas with NO, the maximum in trapping during cycling occurs slightly downstream. Thus more of the catalyst can be used for trapping with NO2 as the NO (X) source. The decreased release during catalyst regeneration is similarly explained; with more being released at the very inlet, there is more residence time and therefore contact with downstream Pt sites, but more importantly more interaction between reductant and stored NO (X) . NH3 and N2O measurements support this conclusion.