Samples of Ba beta-Al2O3 with the stoichiometries BaAl9.O-2(14.8) to BaAl14O22 were prepared by a solid state route from Al2O3 and BaCO3. The activity of Al2O3 was determined in each of the samples of Ba beta-Al2O3 and for samples of a mixture of Al2O3 and phase I Ba beta-Al2O3 with the composition BaAl14O22 and BaAl2O4 mixed with phase II Ba beta-Al2O3 having a composition of BaAl9.2O14.8. In all cases the activity of Al2O3 for the mixtures or compounds was determined against Al2O3 using the solid electrolyte Na beta-Al2O3 as the membrane. It is shown that the trend exhibited by the BaO-Al2O3 compositions are that exhibited by a solid solution of the two compositions BaAl9.2O14.8 and BaAl14O22