Two pure hydrated alkali borates, Na-6[B4O5(OH)(4)](3)center dot 8H(2)O and K-4[B10O15(OH)(4)], have been synthesized under mild hydrothermal conditions and characterized by single X-ray diffraction, infrared spectra, thermogravimetric analysis, differential thermal analysis, and chemical analysis. The enthalpies of solution of Na-6[B4O5(OH)(4)](3)center dot 8H(2)O and K-4[B10O15(OH)(4)] in 1 mol.L-1 HCl(aq) were measured to be (53.37 +/- 0.09) kJ.mol(-1) and (46.61 +/- 0.15) kJ.mol(-1). With the incorporation of the previously determined enthalpies of solution of H3BO3(s) in 1 mol.L-1 HCl(aq) and of NaCl(s) and KCl(s) in (HCl + H3BO3) aqueous solution, together with the standard molar enthalpies of formation of NaCl(s), KCl(s), H3BO3(s), HCl(aq), and H2O(1), the standard molar enthalpies of formation were found to be -(14093.0 +/- 9.7) kJ.mol(-1) for Na-6[B4O5(OH)(4)](3)center dot 8H(2)O and -(8651.0 +/- 8.1) kJ.mol(-1) for K-4[B10O15(OH)(4)] by solution calorimetry.