Abstract
The 
activity in certain compositions of β‐, β''‐alumina ceramic electrolytes was determined by open‐circuit emf measurements. The 
activity was estimated with the assumptions that the β‐, and β''‐alumina phases may be treated as line compounds belonging to the pseudobinary 
system. These thermochemical parameters are utilized in Part II to assess the stability of the electrolytes in Na/S cells.