A colorimetric determination of the chemical warfare agent Lewisite (2-dichloro(2-chlorvinyl)arsine) has been proposed which is based on a reaction with copper(I) iodide and sodium hydroxide in the presence of pyridine. The sensitivity of the determination is high, showing a red colour (formation of copper acetylide) in an aqueous sample containing 5 mg.l^-1 Lewisite.