Thulium(III) iodide is used as a component of metal halide lamps.

[3] Mercury(II) iodide can be heated with thulium to obtain thulium(III) iodide:[4] The mercury produced in the reaction can be removed by distillation.

[5][4] The thulium(III) iodide hydrate crystallized from solution can be heated with ammonium iodide to obtain the anhydrous form.

[6] Its hydrate can be heated in air to obtain TmOI.

Its anhydrous form can be heated with thulium to obtain TmI2:[7]