TlI can be formed in aqueous solution by metathesis of soluble thallium salt with iodide ion.

[7] Thus, presence of impurities might be responsible for coexistence of the cubic and orthorhombic TlI phases at ambient conditions.

Nanometer-thin TlI films grown on LiF, NaCl or KBr substrates exhibit the cubic rocksalt structure.

Thallium(I) iodide is also used in trace amounts with NaI or CsI to produce scintillators used in radiation detectors.

Small grains were found embedded in mascagnite sourced from fumaroles at Avachinsky, a volcano in Russia's Kamchatka Peninsula that can reach temperatures of 640 °C (1,184 °F).