Two pure calcium borates of 2CaO center dot 5B(2)O(3)center dot 5H(2)O (1) and beta-4CaO center dot 5B(2)O(3)-7H(2)O (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 cm(3) of 1 mol center dot dm(-3) HCl(aq) were measured to be -(60.30 +/- 0.78) kJ center dot mol(-1) and -(158.63 +/- 0.72) kJ center dot mol(-1), respectively. The molar enthalpies of solution of H3BO3(s) in 2.00 cm(3) of 1 mol center dot dm(-3) HCl(aq) and of CaO(s) in 2.00 cm(3) of (HCl + H3BO3) aqueous solution were measured to be (21.84 +/- 0.05) kJ center dot mol(-1) and -(188.59 +/- 0.42) kJ center dot 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 center dot mol(-1) and -(377.2 +/- 1.9) kJ center dot 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(1), the standard molar enthalpies of formation were calculated as -(9457.9 +/- 8.3) kJ center dot mol(-1) for (1) and -(11578.7 +/- 8.9) kJ center dot mol(-1) for (II), respectively.