Trimethylsulfoxonium iodide is a sulfoxonium salt with the formula (CH_)_S(O)I. It is a white to off-white crystalline solid, highly hygroscopic, and serves as a stable, non-explosive source of the dimethyloxosulfonium methylide (Corey_Chaykovsky reagent) upon treatment with a strong base such as sodium hydride or potassium tert-butoxide. This reagent is primarily employed in organic synthesis for the one-carbon homologation of carbonyl compounds, converting ketones and aldehydes into epoxides (methylenation), and for the cyclopropanation of activated alkenes. It is also widely used in the synthesis of heterocycles, natural products, and pharmaceutical intermediates due to its ability to transfer a methylene group under mild conditions. As a versatile synthetic building block, it is a standard tool in both academic and industrial laboratories for constructing carbon-carbon bonds. Its stability and ease of handling make it a preferred alternative to diazomethane in many transformations. Proper storage under inert atmosphere is recommended to prevent moisture absorption.