The decomposition of 1,2-dichloroethane on polycrystalline copper has been studied using a microreactor. The reaction is found to have an activation energy of 81 +/- 5 kJ mol(-1) generating gaseous ethene and chemisorbed chlorine. The reaction terminates on completion of a monolayer of chemisorbed chlorine and is followed by a much slower reaction. The rate limiting step is thought to be C2H4Cl2(phys) -> C2H4Cl(ads) + Cl-(ads). The reaction is compared with a UHV study of the same molecule on Cu(111) and the possibility of a negative ion transition state is discussed.