The effect of electric field strength in a cylindrical corona discharge reactor to remove toluene from nitrogen and air is investigated using various wire-cathode diameters in the temperature range from room temperature to 400degreesC. When a thicker cathode is used at elevated temperatures, the required higher voltage and the accompanying gas expansion result in an increase in electron energy. When the reactor temperature is gradually elevated, the apparent removal efficiency decreases with temperature in the temperature range below 100degreesC because of the gas expansion. However, this tendency is reversed in the range above 300degreesC. Without the effect of gas expansion the removal efficiency per residence time indicates that the reaction rate to remove toluene increases with temperature from low temperature up to 400degreesC. In actual application to hot gas cleaning the use of a thicker cathode at high temperature such as 400degreesC without any cooling is recommended for high energetic efficiency.