A highly toxic colourless oily liquid, C₁₁H₂₆NO₂PS; r.d. 1.01; m.p. –50°C; b.p. 298°C. It is an organophosphorus compound and is probably one of the most toxic of all nerve agents. VX was discovered in 1952 at the Porton Down research station in Wiltshire. VX is the best known of the V-series of nerve agents (the V denotes very high persistence in the environment). Other, less well known, members of the series include VG and VM, which are both organophosphorus compounds related to VX.