Phosphonium iodide is commonly used as storage for phosphine[2] and as a reagent for substituting phosphorus into organic molecules.

[3] Phosphonium iodide is prepared by mixing diphosphorus tetraiodide (P2I4) with elemental phosphorus and water at 80 °C and allowing the salt to sublime.

[4][5] Its crystal structure has the tetragonal space group P4/nmm, which is a distorted version of the NH4Cl crystal structure; the unit cell has approximate dimensions 634×634×462 pm.

[1] It oxidizes slowly in air to give iodine and phosphorus oxides; it is hygroscopic[4] and is hydrolyzed into phosphine and HI:[8] Phosphine gas may be devolved from phosphonium iodide by mixing an aqueous solution with potassium hydroxide:[9] It reacts with elemental iodine and bromine in a nonpolar solution to give phosphorus halides; for example: Phosphonium iodide is a powerful substitution reagent in organic chemistry; for example, it can convert a pyrilium into a phosphinine via substitution.

[3] In 1951, Glenn Halstead Brown found that PH4I reacts with acetyl chloride to produce an unknown phosphine derivative, possibly CH3C(=PH)PH2·HI.