LaI3:Ce3+ has the smallest band gap in the LaX3:Ce3+ (X=F, Cl, Br, I) series and hence a potentially higher scintillation yield. The luminescence properties of LaI3:Ce were investigated by means of optical, X-ray, and γ-ray excitation. Unexpectedly, the compound is a poor scintillator at room temperature but presents good scintillation properties for temperatures below 100 K where Ce3+ emission is observed peaking at 452 and 502 nm with a yield of ˜16 000 photons/MeV. The drastic thermal quenching of luminescence is explained by the proximity of the Ce3+ lowest 5d excited state to the host conduction band. Autoionization of Ce3+ therefore prevents any scintillation effect at room temperature. The positions of Ce3+ levels relative to the host bands were estimated.