Three isotypic polyborate cluster compounds for Na-5[AlB24O34(OH)(12)]center dot 5.5H(2)O, Na-5[CrB24O34(OH)(12)]center dot 5H(2)O and Na-5[GaB24O34(OH)(12)]center dot 3H(2)O have been synthesized and characterized by EDS, XRD, FT-IR, TG techniques and chemical analysis. The molar enthalpies of solution of Na-5[AlB24O34(OH) (12)]center dot 5.5H(2)O (s) and Na-5[CrB24O34(OH)(12)]center dot 5H(2)O (s) in 1 mol center dot dm(-3) HCl ( aq) were measured to be (58.27 +/- 0.15) kJ center dot mol(-1) and (130.99 +/- 0.33) kJ center dot mol(-1), respectively. The molar enthalpies of solution of AlCl3 center dot 6H(2)O (s) and CrCl3- center dot 6H(2)O (s) in (HCl + H3BO3 + NaCl) (aq) were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3 (s) in 1 mol center dot dm(-3) HCl (aq), together with the standard molar enthalpies of formation for AlCl3 center dot 6H(2)O (s)/ CrCl3 center dot 6H(2)O (s), NaCl(s), H3BO3 (s), HCl (aq) and H2O (l), the standard molar enthalpies of formation of (21316.0 +/- 19.3) kJ.mol(-1) for Na-5[CrB24O34 (OH) 12] 5H(2)O (s) and (21669.7 +/- 19.3) kJ.mol(-1) for Na-5[AlB24O34(OH)(12)] center dot 5.5H(2)O (s) at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycles. The molar enthalpy of formation of Na-5[GaB24O34(OH)(12))] center dot 3H(2)O was estimated as 20662.9 kJ center dot mol(-1) by using a group contribution method on the basis of these measured results. (C) 2014 Elsevier Ltd. All rights reserved.