The performance of NiO, MnO2, CeO2, Fe2O3, and CuO catalysts on alumina in removing toluene from a gas stream was studied in a plasma catalysis system. The NiO catalyst performed better than the other catalysts, generating more toluene-destroying oxygen species by decomposing ozone. The optimum nickel loading in the NiO/gamma-Al2O3 catalyst was approximately 5 wt%, close to the monolayer dispersion threshold of NiO on gamma-Al2O3. The presence of water vapor had a negative effect on catalytic performance due to its quenching of high speed electrons and its competition with toluene for adsorption sites. Water vapor also reduced the outlet ozone concentration by inhibiting the production of key intermediate in the ozone formation process.