Influence of water vapor on the oxidative decomposition of nitrobenzene in a dielectric barrier discharge (DBD) plasma reactor was investigated, where a sintered metal fibers (SMF) sheet modified with MnO x and CoO x has been used as the inner electrode. The designed in-plasma catalytic reactor in the presence of water vapor showed improved conversion and selectivity to total oxidation, which may be due to the formation of a strong oxidant hydroxyl radical. Optimization of various parameters like concentration, flow rate and applied energy has been carried out and it has been observed that CoO x supported SMF electrode showed better performance, especially for low concentrations of organic compound. In situ decomposition of ozone on the surface of the transition metal oxide modified SMF may also have favored better performance of the plasma reactor. © 2011 Elsevier B.V.