We report enthalpies of formation from the elements and oxides, Delta H-f(o) = -(26195.31 +/- 34.46) kJ . mol (1) and Delta H-f,H-ox = -(1402.09 +/- 33.36) kJ . mol(-1), for crystalline [Al-13(mu(3)-OH)(6)(mu(2)-OH)(18)(H2O)(24)](NO3)(15)center dot 11H(2)O((cr)) (flat-Al-13) measured using aqueous solution room temperature calorimetry. This material is a nitrate salt of a polynuclear aluminum hydroxide ion with thirteen aluminum metal centers, Al-13(15+). Unlike the epsilon-Al-13(7+) Keggin ion, it contains solely octahedrally coordinated aluminum in a more flat topology. We also report heat capacities from temperatures of (2 to 300) K on this flat-Al-13 nitrate using a Quantum Design Physical Property Measurement System (PPMS). These measurements were used to calculate the standard molar entropy and subsequently the free energy of formation for this material. We observed that during the heat capacity measurements, the flat-Al-13 sample underwent a second order phase transition near T = 214 K. (C) 2015 Elsevier Ltd. All rights reserved.