Two pure calcium borates of 2CaO·5B2O3·5H2O (I) and β-4CaO·5B2O3·7H2O (II) have been synthesized under hydrothermal condition and characterized by X-ray diffraction, infrared spectra, and thermogravimetric analysis as well as by chemical analysis. All the enthalpies of solution were measured at 298.15 K by using a heat conduction microcalorimeter. The molar enthalpies of solution of (I) and (II) in 2.00 cm3 of 1 mol·dm-3 HCl(aq) were measured to be −(60.30 ± 0.78) kJ·mol-1 and −(158.63 ± 0.72) kJ·mol-1, respectively. The molar enthalpies of solution of H3BO3(s) in 2.00 cm3 of 1 mol·dm-3 HCl(aq) and of CaO(s) in 2.00 cm3 of (HCl + H3BO3) aqueous solution were measured to be (21.84 ± 0.05) kJ·mol-1 and −(188.59 ± 0.42) kJ·mol-1, respectively. The enthalpies change of borates (I) and (II) formation from the reaction of reagents in the solid phase, −(98.3 ± 1.3) kJ·mol-1 and −(377.2 ± 1.9) kJ·mol-1 were calculated respectively on the basis of the appropriate thermochemical cycles. From these data and the standard molar enthalpies of formation of CaO(s), H3BO3(s), and H2O(l), the standard molar enthalpies of formation were calculated as −(9457.9 ± 8.3) kJ·mol-1 for (I) and −(11578.7 ± 8.9) kJ·mol-1 for (II), respectively.