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Abstract: In 1.30BaO·6Al2O3:Eu and 0.82BaO·6Al2O3:Eu with β′- or β-alumina structure respectively, prepared in air, the doped Eu3+ ions were partially reduced to Eu2+ ions. X-ray powder diffraction patterns and photoluminescent spectra were employed to characterize the compound structures and to detect the existence of Eu2+ ions, respectively. The reduction mechanism of Eu3+→Eu2+ was proposed to be due to the existence of negative defects (e.g. ) in these compounds. This is the first observation of the reduction of Eu3+→Eu2+ in non-stoichiometric compounds prepared in an oxidizing atmosphere. And the luminescent properties of Eu2+ in the compounds of xBaO·6Al2O3:Eu2+ (x=0.82, 1.00, 1.30) showed similar behavior due to their similarity in crystal structures. [Copyright &y& Elsevier]