Gaseous TCE (Trichloroethylene) was decomposed by 185 nm wavelength UV (ultraviolet) lamps and studies were made on the correlation between the residence time and the decomposition percentage, among others. A single UV lamp test reactor was used for the first series of tests. Gaseous TCE was able to be decomposed 100% within a very short residence time. However, the gas following the TCE decomposition contained, besides theoretical final products (hydrogen chloride and carbon dioxide), numerous by-products. Many of these by-products were able to be detoxicated by a sodium hydroxide solution. Increasing of the residence time yielded almost 100% CO2 (carbon dioxide) in terms of carbon balance. Identical tests were also done using a multi-UV lamp test reactor, but as there were differences between it and a single UV lamp version (e.g., reactor volume, number of lamps, and lamp wattage) the decomposition performance (CO2 generation percentage) was evaluated using the electric energy per TCE unit [J/g-(TCE)] as the parameter. The decomposition performance by both apparatus was almost identical.