The sodium oxide activity established by the equilibrium between Na2ZrO3(s) and ZrO2(s) is subject to a continual change upon varying the composition of the phase mixture. This is revealed from the results of a solid state electrochemical approach that allows sensitive differentiation between univariance and bivariance of the phase equilibrium under consideration, thus indicating the occurrence of a noticeable sodium nonstoichiometry presumably in sodium zirconate. (c) 2006 Elsevier Ltd. All rights reserved.