syn-Propanethial S-oxide (or (Z)-propanethial S-oxide), a member of a class of organosulfur compounds known as thiocarbonyl S-oxides (formerly "sulfines"),[2] is a volatile liquid that acts as a lachrymatory agent (triggers tearing and stinging on contact with the eyes).

The chemical is released from onions, Allium cepa, as they are sliced.

A specific sulfenic acid, allicin or 1-propenesulfenic acid, is rapidly rearranged by another enzyme, the lachrymatory factor synthase (LFS) to give syn-propanethial S-oxide.

[3] Vapors from this volatile liquid induces tearing.

[5] Propanethial S-oxide forms a stable thiosultone dimer, trans-3,4-diethyl-1,2-dithietane 1,1-dioxide.