The luminescence properties of Sm 2 + -activated stoichiometric and non-stoichiometric Ba 2 B 5 O 9 Cl were investigated from 16 to 450K. In stoichiometric Ba 2 B 5 O 9 Cl, the emission spectra of Sm 2 + are composed of 4f 6 ->4f 6 transitions over the 16-450K temperature range studied. Luminescence was observed from Sm 2 + in four inequivalent cationic sites at 16K and two inequivalent sites at room temperature. In the non-stoichiometric compound, the emission is characterized by 4f 6 ->4f 6 transitions at low temperature and 4f 5 5d 1 ->4f 6 transitions at high temperature. The Sm 2 + doped into the Ca 2 + and Sr 2 + analogues in the M 2 B 5 O 9 Cl series shows the broad band 4f 5 5d 1 ->4f 6 luminescence in both the stoichiometric and non-stoichiometric compounds due to the increased ligand field caused by the smaller ionic radii of the metal ions.