The reactivity of NOx over two prototype catalysts has been measured in a new bench reactor for characterizing plasma–catalyst systems that allows for in–situ post–analysis of any species which may have adsorbed on the catalyst. In these initial studies without a plasma, NO2 was used to mimic the NOx output of a plasma reactor in a blended feedstream that mimics diesel exhaust. The baseline performance of the catalysts was measured as a function of temperature, hydrocarbon concentration, hydrocarbon type, and water content, usually at a space velocity of 29,000 h–1. Performance was assessed in terms of the percent conversion of the incoming NO2 to desirable non–NOx N–containing species. For the better of the two catalysts the conversion without water present peaked in the 30–40% range between 125°C and 175°C using a propene/propane mixture of hydrocarbons in a 10:1 C1:N ratio. Experiments with NO as the NOx component yielded very poor activities. In studies at 150°C without water using nine different hydocarbons that may be found in diesel exhaust, propene (C3H6) and normal butene (C4H8) gave the best NOx conversion activity. However, in the presence of water, this activity was largely suppressed. This work makes clear that the synergistic adsorption and reactivity of NO2 with active hydrocarbons must compete effectively with water adsorption in any catalyst that will be successful in a plasma–catalyst system.