Lanthanum(III) iodide can be synthesised by the reaction of lanthanum metal with mercury(II) iodide:
:2 La + 3 HgI → 2 LaI + 3 Hg
It can also be prepared from the elements, that is by the reaction of metallic lanthanum with iodine:
:2 La + 3 I → 2 LaI
While lanthanum(III) iodide solutions can be generated by dissolving lanthanum oxide in hydroiodic acid, the product will hydrolyse and form polymeric hydroxy species:
:LaO + 6 HI → 2 LaI + 3 HO → further reactions