The thermal degradation of BaMgAl10O17:Eu2+ (BAM) was investigated by forcibly annealing BAM particles in air at temperatures of 500-1300degreesC. Rietveld analysis with powder X-ray diffraction revealed novel lattice deformations in the thermally degraded BAM. The lattice constants c and a changed monotonically as annealing temperature increased, while the unit-cell volume V showed no change with annealing up to 900degreesC, then abruptly expanded with 1100degreesC annealing. Furthermore, peak profile analysis revealed increasing lattice strain being induced in the host lattice. We suggest that the main cause of the thermal degradation of BAM is due to generation of a lattice vacancy like the Schottky defect. (C) 2004 The Electrochemical Society.