Abstract It is shown that Hong's NASICON solid solution range is intrinsically stable for T > 600 K. With the help of electrochemical and calorimetric experiments, complete thermochemical information of Na 1+ x Zr 2Si x P 3− x O 12 (0 ≦ x ≦ 3) compositions has been elucidated, i.e., formation enthalpies, standard entropies, and molar volume (1.5–1000 K). For the purpose of a precise evaluation of the electrochemical values the specific heats of ZrP 2O 7 and Na 2ZrSi 2O 7 have also been determined. A thermodynamic mixture model is constructed showing that the solid solution is entropy stabilized. The energetic interaction has a destabilizing influence. The zero-point entropy and the excess entropy at higher temperatures suggest a partial ordering ( Si P ) at higher temperatures. A miscibility gap, however, which is predicted for T ≤ 600 K by the model, could not be found due to the slow kinetics or because of a simultaneous phase transformation. The obtained values clearly show that NASICON is thermodynamically unstable with respect to a reaction with elementary sodium. The instability increases with increasing P content.