A red solid, it is too unstable to be stored for long periods of time; it is, nevertheless, commercially available.

[2] It is widely used in organic chemistry for converting alcohols to alkyl iodides and also serves as a powerful reducing agent.

The P-I bond is also weak; PI3 is much less stable than PBr3 and PCl3, with a standard enthalpy of formation for PI3 of only −46 kJ/ mol (solid).

Phosphorus triiodide is commonly used in the laboratory for the conversion of primary or secondary alcohols to alkyl iodides.

Often the PI3 is made in situ by the reaction of red phosphorus with iodine in the presence of the alcohol; for example, the conversion of methanol to give iodomethane:[5] These alkyl iodides are useful compounds for nucleophilic substitution reactions, and for the preparation of Grignard reagents.