The heat capacity investigation of crystalline pentasodium zirconium tris(phosphate) was carried out in a vacuum adiabatic calorimeter between 7 and 340 K and in a differential scanning calorimeter of the heat bridge type between 330 and 620 K. Between 389 and 424 K, an isostructural solid-to-solid phase transition of Na5Zr(PO4)(3), has been found, the nature of which is connected with a centering of off-centered zirconium atoms in octahedral sites and an occupation transfer between sodium sites in the structure. The results were used to calculate the characteristics of the phase transition and the thermodynamic functions of Na5Zr(PO4)(3): the transition temperature T(trs)degrees, enthalpy of transition Delta(trs)Hdegrees, entropy of transition Delta(trs)Sdegrees; enthalpy Hdegrees(T) - Hdegrees(0), entropy Sdegrees(T) and Gibbs function Gdegrees(T) - Hdegrees(0) over the range from 0 to 620 K. From hydrofluoric acid solution microcalorimetry, the enthalpy of solution of Na5Zr(PO4)(3) at 298.15 K has been determined and the standard enthalpy of formation has been derived. By combining the data obtained by the two techniques, the Gibbs function of formation of Na5Zr(PO4)(3) at 298.15 K has been calculated. (C) 2003 Elsevier Science B.V. All rights reserved.