We investigated the intercalation of water into BaMgAl10O17:Eu2+ (BAM), a blue phosphor that is used in plasma display panels. The adsorption and desorption characteristics of water with BAM have hysteresis; showing that water is intercalated into BAM. Using thermal analysis techniques, we suggested that water hydrated to barium ions caused oxidation. We found that the water intercalated into BAM played an important role in the oxidation of Eu2+ between 450 and 600, and contributed to a 10% degradation of luminance. In contrast, oxidation due to oxygen is a principal factor in degradation above 600 through baking process in air.