A thiosulfoxide or thiothionyl compound is a chemical compound containing a sulfur to sulfur double bond, with the formula (R−)(R'−)S=S, where R and R' represent any group (typically fluorine, chlorine, alkoxy, alkyl, aryl or other organyl residues. The thiosulfoxide has a molecular shape known as trigonal pyramidal. Its coordination is also trigonal pyramidal. The point group of the thiosulfoxide is C_s. A 1982 review concluded that there was as yet no definitive evidence for the existence of stable thiosulfoxides ^[1] which can be attributed to the double bond rule which states that elements of period 3 and beyond do not form multiple bonds. The related sulfoxides of the type (R−)(R'−)S=O are very common. Many compounds containing a sulfur-sulfur double bond have been reported in the past although only a few verified classes of actually stable compounds exist, closely related to thiosulfoxides.

Sulfur-sulfur double bonds can be stabilized with electron-withdrawing groups in so-called thionosulfites of the type (R−O−)(R'−O−)S=S. These compounds can be prepared by reaction of diols with disulfur dichloride. Sulfur halides such as disulfur dichloride, Cl−S−S−Cl, can convert to the branched isomer thiothionyl chloride, Cl₂S=S; disulfur difluoride exists as an equilibrium mixture with thiothionyl fluoride, F₂S=S, which is thermodynamically more stable. These disulfide isomerizations are occasionally studied in silico.^[2]

N-(Thiosulfinyl)amines of the type R−N=S=S are another group of stable compounds containing a S=S bond. The first such compound was prepared in 1974 reaction of the nitroso compound N,N-dimethyl-p-nitrosoaniline with tetraphosphorus decasulfide. Heating to 200 °C extrudes sulfur in this compound and forms the corresponding azo compound. Disulfur monoxide S=S=O is stable at 20 °C for several days.

Occasionally thiosulfates are depicted as having a S=S unit but the sulfur-sulfur bond in it is in fact a single bond.

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