We have prepared a new salt of the solvatochromic [Ru(bpy)(CN)4]2- anion bis(triphenylphosphine)iminium 2,2′-bipyridyltetracyanoruthenate, ((PPN)2[Ru(bpy)(CN)4]), that reversibly sorbs water and changes color from purple to yellow when it is exposed to humid air. Solid samples of anhydrous (PPN)2[Ru(bpy)(CN)4] reversibly hydrate in humid air, first to (PPN)2[Ru(bpy)(CN)4]-H2O and then to (PPN)2[Ru(bpy)(CN)4]·15H2O. A single-crystal X-ray study of (PPN)2[Ru(bpy)(CN)4]·2CH 3CN·2(CH3CH2) 2O·2H2O shows that this novel humidity sensor has large channels in the structure that allows sorbed water molecules to reversibly interact with the cyanide ligands of the chromophore. The hydration changes that result from exposure of the solid in the 0 to 66% relative humidity range cause the MLCT visible absorption bands to shift dramatically to higher energy. These hydration changes also result in large shifts in the v(CN) region of the infrared spectrum; these changes indicate significant H-bonding interactions between sorbed water molecules and the dianionic chromophore occur in the crystal lattice. The hydration changes in the 66 to 100% relative humidity range cause large intensity changes in a visible photoluminescence band. This emission intensity increase is accompanied by small wavelength shifts in absorption and emission and in the v(CN) region of the infrared spectrum. These changes are consistent with further oligomerization of the water molecules that initially formed hydrogen-bonding interactions to the cyanide ligands.