1,4-diazoniabicyclo [2.2.2]octane diiodide monohydrate was synthesized by protonation of 1,4-diazabicyclo[2.2.2]octane (DABCO) with an excess of concentrated hydroiodic acid in water. It forms colourless rodlike crystals. At room temperature an orthorhombic unit cell, space group Cmc2(1), a = 7.781(2) angstrom, b = 13.010(3) angstrom, c = 11.610(2) angstrom, V = 1175.3(4) angstrom(3), Z = 4, is observed. Both nitrogen atoms bear a proton. The hydrate water molecule within the compound is statistically disordered on a site besides a crystallographic mirror plane at room temperature. DSC measurements indicate a phase transition at about -10 degrees C. A single crystal X-ray diffraction measurement at 123 K reveals a primitive unit cell, space group Pca2(1), a = 12.8835(2) angstrom, b = 7.6819(1) angstrom, c = 11.4392(2) angstrom, v = 1132.16(3) angstrom(3), Z = 4. The water molecule with its hydrogen atoms is well ordered at this temperature. Two almost linear hydrogen bonds O-H center dot center dot center dot I are formed. The formation of hydrogen bonds is also detected by IR-spectroscopy with O-H stretching frequencies at (nu) over bar = 3403 cm(-1) and (nu) over bar = 3354 cm(-1), respectively.