Two pure hydrated lead borates, Pb4B10O19 center dot 2.5H(2)O and Pb6B11O18(OH)(9), have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Pb4B10O19 center dot 2.5H(2)O and Pb6B11O18(OH)9 in 1 mol dm(-3) HNO3(aq) were measured to be -51.44 +/- 0.18 kJ mol(-1) and -91.70 +/- 0.19 kJ mol(-1), respectively. With the incorporation of the previously determined enthalpies of solution of H3BO3(s) in 1 mol dm(-3) HNO3(aq) and of PbO(s) in (HNO3+ H3BO3)(aq), together with the use of the standard molar enthalpies of formation for Pb0(s). H3BO3(s) and H2O(1), the standard molar enthalpies of formation of -8231.4 +/-8.6 kJ mol(-1) for Pb4B10O19 center dot 2.5H(2)O and -9967.5 +/- 9.8 kJ mol(-1) for Pb6B11O18 center dot 4.5H(2)O were obtained on the basis of the appropriate thermochemical cycles. (C) 2011 Elsevier B.V. All rights reserved.