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A simulated diesel exhaust is treated with a nonthermal plasma discharge under steady state conditions. The plasma effluent is then passed through a sodium zeolite-Y (NaY) catalyst followed by a platinum oxidation catalyst. Detailed FTIR measurements of gas composition are taken before, between, and after the treatment stages. The plasma discharge causes oxidation of NO primarily to NO2, with methyl nitrate and nitric acid byproducts. At the same time, HC is partially oxidized, creating species such as formaldehyde, acetaldehyde, CO and other partial oxidation products. When this mixture passes over the NaY catalyst, part of the NOx is reduced to N2, with the remainder primarily in the form of NO. Methyl nitrate decomposes to form methanol and NOx, and nitric acid is consumed. There is little HC conversion on this catalyst. Small quantities of HCN and N2O are formed. When the mixture then passes over the platinum catalyst, further NOx conversion occurs.