Catalytic effects of metal oxides in nonthermal plasma chemical processing of hazardous air pollutants (HAPs) are discussed, relevant to their activities for the oxidation of HAPs in nonthermal plasma media and their selective control of active oxygen species derived from background O-2. In ferroelectric packed-bed reactors, the oxidation power of barium titanate (BaTiO3) is not strong enough to oxidize HAPs and their carbon intermediates to CO2. Only nitrous oxide (N2O) was formed from background N-2 and lattice oxygen atoms in BaTiO3. The catalytic effect of BaTiO3 is negligible under aerated conditions. On the other hand, ozone (O-3) is formed from background O-2 in much higher concentrations in a silent discharge plasma reactor. Manganese dioxide (MnO2)-catalyzed decomposition of 03 promotes decomposition of benzene, which is less reactive titan trichloroethylene and tetrachloroethylene. The acceleration of benzene consumption rate is ascribed to the promotion of its oxidative decomposition by the triplet oxygen atom. Catalytic control of in situ active oxygen species could be one of the most effective approaches to increase the energy efficiency of the nonthermal plasma reactor and to achieve the complete oxidation of the carbon atoms in HAPs.