Light emission from the first excited state of Mn4+ ions in the orthorhombic red phosphor K2SnF6 center dot H2O:Mn4+ is reported. The hydrate phosphor K2SnF6 center dot H2O:Mn4+ is synthesized from metallic tin by wet chemical etching in aqueous HF/KMnO4 solution. Vacuum evacuation, immersion in methanol, or heating in air leads to partial dehydration of this hydrate phosphor. The dehydration-induced lattice deformation results in the splitting of the Mn4+-related emission lines, together with an enhancement of the zero-phonon line emission intensity. The partially dehydrated state returns to the rehydrated state in air at room temperature within a day. Such unique photoluminescent properties have not been observed previously in any other phosphor or luminescent material. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3532784] All rights reserved.