Solid-state electrochemical and XRD studies reveal that the phase relation between Na2ZrO3( s) and ZrO2( s) is governed by the existence of non-stoichiometric Na-modified cubic ZrO2. It leads to the splitting of the nominal Na2ZrO3/ZrO2 ( NZO) equilibrium system into two systems: Na2ZrO3=Zr1-xmax Na-xmax O2-1.5xmax and Zr1-xmin Na-xmin O2-1.5xmin/ZrO2. The causes extended single-phase region in between the thermodynamic stability in the temperature interval 400 degrees-600 degrees C to change steadily upon varying the composition of the phase mixture. As a consequence, previous data on the thermodynamic stability of the NZO system have to be considered with caution.