Two new hydrated alkali borates, Li-8[B16O26(OH)(4)]center dot 6H(2)O and Cs2B7O9(OH)(5), have been synthesized under mild hydrothermal conditions and characterized by X-ray diffraction, infrared spectra, thermogravimetric analysis, differential thermal analysis, and chemical analysis. The enthalpies of solution of Li-8[B16O26(OH)(4)]center dot 6H(2)O and Cs2B7O9(OH)(5) in 1 mol.dm(-3) HCl(aq) were measured to be -(7.53 +/- 0.25) kJ.mol(-1) and (64.38 +/- 0.15) kJ.mol(-1). With the incorporation of the previously determined enthalpies of solution of H3BO3(S) in 1 mol.dm(-3) HCl(aq) and of LiCl center dot H2O(s) and CsCl(s) in (HCl + H3BO3) aqueous solution, together with the standard molar enthalpies of formation of LiCl center dot H2O(s), CsCl(s), H3BO3), HCl(aq), and H2O(1), the standard molar enthalpies of formation were found to be -(15943 +/- 13) kJ.mol(-1) for Li-8[B16O26(OH)(4)]center dot 6H(2)O and -(6099 +/- 6) kJ.mol(-1) for Cs2B7O9(OH)(5) by solution calorimetry.