Until now the thermodynamic stability of Na-beta (alpha + beta/beta + beta "/beta)-alumina has been determined exclusively by measuring the potential on appropriate solid electrolyte galvanic cells and evaluating the results according to the Nernst equation. In this paper the validity of Nernst’s law for these measurements is critically discussed in the light of the electronic conductivity data published so far for Na-beta-alumina. The following conclusions have been drawn : (i) the stability measurements may be adversely affected by electronic conduction when the sample is exposed to a comparatively high oxygen pressure, e.g., air, and (ii) the recently reported findings on the p-electron conduction parameter, which contradicted the literature data, describe the electronic properties of Na-beta-alumina more accurately than the previous results.