Much work has been done on the application of plasmas to the treatment of NOx from power plants. In power plant applications, the purpose of the plasma is to oxidize NO to NO2, and eventually to nitric acid. The desired products, in the form of ammonium salts, are then obtained by mixing ammonia with the formed acids. Some form of scrubbing is required to collect the final products. For applications to the treatment of exhausts from cars and trucks, it is very important to make a distinction between NO removal by chemical oxidation and NO removal by chemical reduction. To avoid the need for scrubbing of plasma processing products, the desired method of NO removal is by chemical reduction; i.e. the conversion of NO to benign gaseous products like N2. This paper will discuss the results of an extensive series of experiments aimed towards understanding the effect of gas composition on the NOx conversion chemistry in a plasma. The NOx conversion chemistry in the presence of the individual components, such as N2, O2, H2O, and CO2, as well as the mixture of these, will be presented. We will show that, in a lean-burn gasoline or diesel engine exhaust, the main effect of the gas-phase reactions in a plasma is the oxidation of NO to NO2 and nitric acid. To implement the reduction of NOx to N2 in the highly oxidizing environment of a lean-burn engine exhaust, it will be necessary to prevent the formation of acid products and combine the plasma with another process that can chemically reduce NO2 to N2.