A dielectric barrier discharge device has been built to test nonthermal plasma discharges for simulated diesel exhaust NOx removal. The device has also been tested with selected catalysts located after the plasma. Emissions are measured by conventional automotive emission analyzers, plus FTIR.Dielectric barrier discharges without catalyst convert input NO to a mix of NO2, HONO, HNO3, and organic nitrates. At 30 J/l energy deposition, approximately 26% of the input NO is “lost”. Some of the hydrocarbon input is converted to a variety of species, including CO, CO2, aldehydes, and alcohols.A Cu-ZSM catalyst after the plasma device eliminates the apparent NOx conversion seen with the bare plasma. This indicates that the apparent NOx conversion of the bare plasma is actually conversion to some (unmeasured) species which can be reconverted to NOx by the Cu-ZSM catalyst.Placing a proprietary catalyst within the plasma results in significant NOx conversion. In this case, 56% of the input NO is not found in measured species after the plasma-catalyst system. Remaining NOx is primarily NO. A Cu-ZSM catalyst after this system does not degrade NOx efficiency.Chemiluminescent NOx analyzers will measure significantly lower NOx conversion than the true value when formaldehyde is present.