The effect of water vapor on benzene decomposition in air was investigated using a nonthermal-discharge plasma reactor packed with ferroelectric materials. The conversion of benzene was found to decrease with an increase of water concentration. on the other hand, the selectivity to CO2 in the decomposition products increased with an increase of water concentration. A comparison between the benzene conversion to CO and CO2 suggested that CO formation was suppressed by water to a greater extent than was CO2 formation. N2O formation also decreased with an increase of water concentration. These results suggest that the activity of oxygen species responsible for the formation of CO and N2O is reduced by water.