Trimethylsulfonium iodide is primarily used as a methylating agent in organic synthesis, facilitating the transfer of methyl groups to various substrates. It finds application in the preparation of pharmaceuticals, where it is employed to methylate amines, alcohols, and other nucleophiles, enhancing the biological activity or stability of the compounds. Additionally, it is utilized in the synthesis of specialty chemicals, including dyes and agrochemicals, where precise methylation is required. Its role in polymer chemistry is also notable, as it aids in modifying polymers to achieve desired properties. The compound’s effectiveness in methylation reactions makes it a valuable reagent in research and industrial processes.