CH3AsCl2, a colorless liquid with a sharp irritating odor. Melting point, −59°C; boiling point, 134.5°C ; density, 1.84 g/cm3 at 20°C . Methyldichlorarsine is poorly soluble in water but readily soluble in organic solvents. It is hydrolyzed by water to form toxic methylarsinoxide and is readily oxidized to nontoxic methylarsenic acid. It reacts with H2S to produce methylarsinesulfide, which is insoluble in water (this reaction is used in the qualitative detection of methyldichlorarsine).
Methyldichlorarsine irritates the upper respiratory tract and exhibits a systemic and blistering effect (blisters appear upon contact of 3–5 mg methyldichlorarsine per sq cm of skin). It was used to a limited extent as a poison substance during World War I (1914–18). Dimercaprol and its derivatives are used in treating cases of methyldichlorarsine poisoning.