AbstractEmission and excitation spectra of BaS:Bi3+ phosphors are measured at 300, 80, and 6 K. I n the excitation spectra the A and C excitation bands are observed. In the emission spectra only one band is observed which is assigned to the transition 3A1u → 1A1g on the basis of the measurement of the time‐resolved emission spectra and the decay constant of the phosphorescence. The diffused reflection spectra and the magnetic effect on the phonon structure of the emission band are also measured at 80 and 4.2 K, respectively. The former is compared with the excitation spectra. The reason why the emission band 3T1u → 1A1g cannot be observed at low temperatures is explained by the fact that the probability of the tunnel effect 3T1u → 3A1u is larger than that of the luminescent transition 3T1u → 1A1g.