A microwave plasma system using a 2.45 GHz magnetron was applied to the decomposition of volatile organic compounds such as toluene and trichloroethylene. Designed for producing plasma at atmospheric pressure, this microwave plasma system consists of a magnetron detached from a household microwave oven, a directional coupler, a three-stub tuner, a tapered waveguide, and plasma flame section where a quartz tube with a nozzle is located. In this system, the organic compounds can be decomposed by thermal incineration as well as by reactions with various active species formed during plasma discharge. The effect of feed gas flow rate on the decomposition was significant due to the decrease in the gas temperature, but the initial concentration in the range of 210-2,100 ppm did not largely affect the decomposition efficiency. The principal byproduct was nitrogen oxides because this system was operated at high temperature. To improve the decomposition of the organic compounds, argon was used as a plasma-assisting gas, together with the air-like feed gas mixture. Large enhancement in the decomposition efficiency was achieved by the use of argon.