VX (IUPAC name O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate) is an extremely toxic substance that has no known uses except in chemical warfare as a nerve agent. It is a tasteless and odorless liquid. As a chemical weapon, it is classified as a weapon of mass destruction by the United Nations in UN Resolution 687. The production and stockpiling of VX exceeding 100 grams per year was outlawed by the Chemical Weapons Convention of 1993.

The VX nerve agent is the best-known of the V-series of nerve agents and is considered an area denial weapon due to its physical properties.

Discovery

Ranajit Ghosh, a chemist at the Plant Protection Laboratories of the British firm Imperial Chemical Industries (ICI), was investigating a class of organophosphate compounds (organophosphate esters of substituted aminoethanethiols). Like Gerhard Schrader, an earlier investigator of organophosphates, Ghosh found that they were quite effective pesticides. In 1954, ICI put one of them on the market under the trade name Amiton. It was subsequently withdrawn, as it was too toxic for safe use. The toxicity did not go unnoticed, and samples of it had been sent to the British Armed Forces research facility at Porton Down for evaluation. After the evaluation was complete, several members of this class of compounds became a new group of nerve agents, the V agents. The best-known of these is probably VX, assigned the UK Rainbow Code Purple Possum, with the Russian V-Agent coming a close second (Amiton is largely forgotten as VG). This class of compounds is also sometimes known as Tammelin's esters, after Lars-Erik Tammelin of the Swedish Defence Research Agency. Tammelin was also conducting research on this class of compounds in 1952, but did not widely publicize his work.