In 1.30BaO·6Al 2 O 3 :Eu and 0.82BaO·6Al 2 O 3 :Eu with β′- or β-alumina structure respectively, prepared in air, the doped Eu 3+ ions were partially reduced to Eu 2+ ions. X-ray powder diffraction patterns and photoluminescent spectra were employed to characterize the compound structures and to detect the existence of Eu 2+ ions, respectively. The reduction mechanism of Eu 3+ →Eu 2+ was proposed to be due to the existence of negative defects (e.g. ONa‴) in these compounds. This is the first observation of the reduction of Eu 3+ →Eu 2+ in non-stoichiometric compounds prepared in an oxidizing atmosphere. And the luminescent properties of Eu 2+ in the compounds of xBaO·6Al 2 O 3 :Eu 2+ (x=0.82, 1.00, 1.30) showed similar behavior due to their similarity in crystal structures.