We have investigated the degradation of BaMgAl10O17: Eu2+ phosphor (BAM) using Eu-151 Mossbauer and fluorescence spectroscopic methods. The Eu-151 Mossbauer measurements show that upon hydration, a significant fraction of divalent europium ions is oxidized. The degradation process induced by the intercalated water molecules into the host lattice appears to be a unique process. It has many features that are both similar to and distinct from those induced by oxidation in air and those caused by discharge during the lamp life. A plausible mechanism of degradation of BAM caused by the intercalated water molecules is discussed. (c) 2005 The Electrochemical Society. All rights reserved.