Two pure microporous materials, Rb[ZnBP2O8] and Cs[ZnBP2O8], have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Rb[ZnBP2O8] and Cs[ZnBP2O8] (s) in 3 mol . dm (3) HCl (aq), of Zn(PO3)(2) (s) in (HCl + H3BO3) (aq), and of RbCl (s)/CsCl(s) in [Zn(PO3)(2) + HCl + H3BO3] (aq) were measured by microcalorimeter at T = 298.15 K, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3 (s) in 3 mol . dm (3) HCl (aq), together with the use of the standard molar enthalpies of formation for Zn(PO3)(2) (s), RbCl (s)/CsCl (s), H3BO3 (s), HCl (aq) and H2O (l), the standard molar enthalpies of formation of -(3198.5 +/- 1.2) kJ . mol (1) for Rb[ZnBP2O8] and -(3212.7 +/- 1.2) kJ . mol (1) for Cs[ZnBP2O8] at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycle. In addition, the molar enthalpy of formation of -(2944.4 +/- 1.2) kJ . mol (1) for [ZnBP2O8] has also been estimated by a group contribution method, which can be used to predict the Delta H-f(m)theta the other similar borophosphates. Moreover, these data have been further used to compare the thermodynamic stability of this series of borophosphates. (C) 2014 Elsevier Ltd. All rights reserved.