The effects of hydrocarbons and water vapor on a combined deNO(x) process, which consists of nonthermal plasma oxidation and V2O5-WO3/TiO2 selective catalytic reduction (SCR), are investigated. Test results show that the fast SCR reaction, which needs equimolar amounts of NO and NO2, plays a dominant role in reducing NOx under relatively low-temperature conditions, i.e., 150-200 degrees C. Under such low-temperature conditions, the oxidation of NO to NO2 induced by nonthermal plasma is useful in controlling NO2 fractions in NOx for the fast SCR reaction. The role of the fast SCR reaction, however, decreases when C3H6 is supplied to the process as simulated hydrocarbons in diesel exhausts. Test results yielded the following conclusions: (1) C3H6 leads to the production of aldehydes in the nonthermal plasma reactor. (2) The NO2 fraction in the SCR reactor decreases as a result of aldehyde production, leading to a diminished role of the SCR reaction. This reduction in NO2 is predominant when water vapor is not added to the test gases. (3) The decreased role of the fast SCR reaction is recovered when water vapor is present in the treated gases, and the optimal NO2 fraction for peak deNO(x) conversion shifts to higher fractions (greater than 0.5), which should be considered in treating diesel exhaust.