[4][5] This orthorhombic structure is typical of the triiodides of the lighter lanthanides (La–Nd), whereas heavier lanthanides tend to adopt the hexagonal bismuth(III) iodide structure.

[3] Lanthanum(III) iodide is very soluble in water and is deliquescent.

[4] Anhydrous lanthanum(III) iodide reacts with tetrahydrofuran to form a photoluminescent complex, LaI3(THF)4, with an average La–I bond length of 3.16 Å.

It is an electride and is best formulated {LaIII,2I−,e−}, with the electron delocalised in a conduction band.

[11] Reduction of LaI2 or LaI3 with metallic sodium in an argon atmosphere at 550 °C gives lanthanum monoiodide, LaI, which has a hexagonal crystal structure.