This paper discusses the location of Eu2+ ions at three possible sites and the excitation mechanism in an efficient blue-emitting phosphor, Ba1-xEuxMgAl10O17, based on vacuum ultraviolet excitation spectra obtained with synchrotron radiation and the first-principles calculation. Especially, the dependence of these properties on Eu concentration was investigated in detail. The luminescence bands are observed at about 2.7 and 4.8 eV, which originate from the Eu2+ ions and the host lattice, respectively. The excitation spectra monitored at the 2.7 eV emission band show two main structures peaking at 4 and 5 eV, as well as a significant absorption band at 6.2 eV. The absorption edge of the host lattice at 6.5 eV is almost independent of Eu concentration. Electronic state calculations for different Eu2+ sites indicate the contribution of the mO site to the 6.2 eV structure. However, in the luminescence spectrum under the excitation at 6.2 eV, the 3.2 eV emission band with an extrinsic characteristic was strongly observed. Therefore, in addition to the mO site, some kind of defects or impurities also contributes to the 6.2 eV structure. Moreover, the theoretical absorption spectra exhibit a strong polarization dependence, which is the cause of a significant sample dependence of excitation properties. (C) 2007 The Electrochemical Society.