Selenosulfides have been prepared by the reaction of selenyl halides with thiols:
The equilibrium between diselenides and disulfides lies on the left:
:RSeSeR  +  RSSR   2 RSeSR'
Because of the facility of this equilibrium, many of the best characterized examples of selenosulfides are cyclic, whereby S-Se bonds are stabilized intramolecularly.  One example is the 1,8-selenosulfide of naphthalene. The selenium-sulfur bond length is about 220 picometers, the average of a typical S-S and Se-Se bond.