(Iodomethyl)triphenylphosphonium iodide is a phosphonium salt used primarily in organic synthesis as a Wittig reagent precursor. Treated with a base, it generates the ylide (iodomethylidene)triphenylphosphorane (Ph₃P=CHI), which reacts with aldehydes and ketones to form β-iodoalkenes. These vinyl iodides serve as key intermediates in the synthesis of pharmaceuticals, agrochemicals, and natural products, enabling further transformations like cross-coupling reactions due to the iodine functionality. It also finds applications in material science for modifying polymers and incorporating iodine-containing groups to enhance properties such as reactivity and conductivity.