Methyldichloroarsine, sometimes abbreviated "MD" and also known as methyl Dick,[1] is an organoarsenic compound with the formula CH3AsCl2.

[2] Focusing on the arsenic center, the molecule geometry is trigonal pyramidal with the Cl-As-Cl and C-As-Cl angles approaching 90° (see image).

In World War I, the German manufacturing method consisted of a three-step reaction beginning with methylation of sodium arsenite: followed by reduction of the disodium monomethylarsonate with sulfur dioxide: subsequently reacting the monomethylarsine oxide thus formed with hydrogen chloride to yield methyldichloroarsine:[4] The As-Cl bonds in MD are susceptible toward nucleophilic attack.

[5] Other symptoms include: dermal burns with vesicle formation; blepharospasm and photophobia.

Convulsions, abdominal pain, coughing, and shortness of breath with damage to the respiratory system can be delayed for about three to five days; hemolysis can also occur.