Luminescence properties and mechanisms of thermal degradation in (Sr,Ba)(3)MgSi2O8:Eu2+ under vacuum-UV-light excitation have been investigated. Sr2.98MgSi2O8:Eu-0.02(2+) and a Ba-substituted composition, Sr2.48Ba0.5MgSi2O8:Eu-0.02(2+), are over 90 and 75%, respectively, as high as commercially used BaMgAl10O17:Eu2+ in photoluminescence peak intensity under 146 nm light excitation. At the same Eu content, Ba-substituted Sr3MgSi2O8:Eu2+ shows much smaller thermal degradation of luminescence than Sr3MgSi2O8:Eu2+. X-ray fine structure measurements show that the thermal degradation is closely correlated to the oxidation of Eu2+ ion into Eu3+ ion, which is suppressed by Ba substitution. (c) 2008 The Electrochemical Society.