At 62 °C and atmospheric pressure, phosphonium iodide sublimates and dissociates reversibly into phosphine and hydrogen iodide (HI). It oxidizes slowly in air to give iodine and phosphorus oxides; it is hygroscopic and is hydrolyzed into phosphine and HI:
Phosphine gas may be devolved from phosphonium iodide by mixing an aqueous solution with potassium hydroxide:
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. In 1951, Glenn Halstead Brown found that PHI reacts with acetyl chloride to produce an unknown phosphine derivative, possibly .