A pure hydrated potassium borate K2B5O8(OH)(.)2H(2)O has been synthesized under mild hydrothermal conditions and characterized by single-crystal X-ray diffraction, XRD, FIF-IR, Raman spectra and DTA-TG. The crystal structure consists of two K-O polyhedra and [B5O8(OH)](2-) polyborate anion. The enthalpy of formation was determined to be -4772.6 +/- 4.0 kJ mol(-1) by solution calorimetry. (c) 2006 Elsevier B.V. All rights reserved.