Abstract
The degradation of β‐ and β''‐alumina electrolytes in Na/S cells is assessed from a thermochemical viewpoint. Two probable schemes for the S‐side degradation are considered: (i) 
and (ii) free 
liberation. The probable degradation mechanism at the Na electrode, considered in this assessment, involves reactions leading to the liberation of metallic Al. The assessment indicates that α‐ and β‐alumina with low 
activity are thermodynamically stable, while the other soda‐rich phases in the 
system are unstable to the 
leaching reactions. On the other hand, thermodynamic stability to the reaction with Na is favored by high 
activity and qualifies soda‐rich β''‐alumina and other soda‐rich compounds as stable phases. Thus, the existence of any phase in the 
system which would be stable at both the Na and the S electrode, is negated by the assessment.