Abstract K β′-alumina is unstable at >1300°C. Mixed alkali β′-alumina has a variable stability depending on the alkali ratio, [K +]/([Na +]+[K +]). For f( β)<[K +]/([Na +]+[K +]), the β′-Al 2O 3 phase decomposes to Kβ-Al 2O 3 0997 0815 V 3 and a solid solution of Na β′-Al 2O 3 and K β′-Al 2O 3. For f( β)=[K +]/([Na +]+[K +], the ceramic consists of K β-Al 2O 3 and Naβ′-Al 2O 3 and for f( β)>[K +]/([Na +]+[K +]), the excess Na + after Na β′-Al 2O 3 dissolves in the β phase, giving Na β-Al 2O 3/K β-Al 2O 3 solid solution and Na β′-Al 2O 3. These sequences were confirmed by measuring the dependence of the c-axis lattice parameters of β- and β′-Al 2O 3 phases on the f(β), and the change of these parameters during the ion-exchange of Na + and K + ions.