NOx reduction with NO2 as the NOx gas in the absence of plasma was compared to plasma treated lean NOx exhaust where NO is converted to NO2 in the plasma. Product nitrogen was measured to prove true chemical reduction of NOx to N-2. With plasma treatment, NO as the NOx gas, and a NaY catalyst, the maximum conversion to nitrogen was 50% between 180 and 230degreesC. The activity decreased at higher and lower temperatures. At 130 C a complete nitrogen balance could be obtained, however between 164 and 227 degreesC less than 20% of the NOx is converted to a nitrogen-containing compound or compounds not readily detected by gas chromatograph (GC) or Fourier transform infrared spectrometer (FT-IR) analysis. With plasma treatment, NO2 as the NOx gas, and a NaY catalyst, a complete nitrogen balance is obtained with a maximum conversion to nitrogen of 55% at 225degreesC. For gamma-alumina, with plasma treatment and NO2 as the NOx gas, 59% of the NOx is converted to nitrogen at 340degreesC. A complete nitrogen balance was obtained at these conditions. As high as 80% NOx removal over gamma-alumina was measured by a chemiluminescent NOx meter with plasma treatment and NO as the NOx gas. When NO is replaced with NO2 and the simulated exhaust gases are not plasma treated, the maximum NO, reduction activity of NaY and gamma-alumina decreases to 26 and 10%, respectively. This is a large reduction in activity compared to similar conditions where the simulated exhaust was plasma treated, Therefore, in addition to NO2, other plasma-generated species are required to maximize NOx reduction.