Sulfides are typically prepared by alkylation of thiols. Alkylating agents include not only alkyl halides, but also epoxides, aziridines, and Michael acceptors.
Such reactions are usually conducted in the presence of a base, which converts the thiol into the more nucleophilic thiolate. Analogously, the reaction of disulfides with organolithium reagents produces thioethers:
Analogous reactions are known starting with Grignard reagents.
Alternatively, sulfides can be synthesized by the addition of a thiol to an alkene in the thiol-ene reaction:
This reaction is often catalysed by free radicals produced from a photoinitiator.
Sulfides can also be prepared by many other methods, such as the Pummerer rearrangement. Trialkysulfonium salts react with nucleophiles with a dialkyl sulfide as a leaving group:
This reaction is exploited in biological systems as a means of transferring an alkyl group. For example, S-adenosylmethionine acts as a methylating agent in biological S2 reactions.
An unusual but well tested method for the synthesis of thioethers involves addition of alkenes, especially ethylene across the S-Cl bond of sulfur dichloride.  This method has been used in the production of bis(2-chloroethyl)sulfide, a mustard gas: