O-mustard (T) is a vesicant chemical weapon, a type of mustard gas, with around three times the toxicity of the original sulfur mustard.

It was developed in England in the 1930s as a thickener for mustard gas to make it more persistent when used in warm climates.

A mixture of 60% sulfur mustard and 40% O-mustard also has a lower freezing point than pure sulfur mustard, and was given the code name HT.

O-mustard is a Schedule I substance under the Chemical Weapons Convention.

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