Equilibrium and metastable-equilibrium phase relationships and gas-species distributions in the systems : (Na1-xLix)-beta"-Al2O3-O2 (x = 0.05, 0.1, 0.2, 0.3 and 0.5) and (Na1-xLix)-beta"-Al2O3-O2-H2O (x = 0.05) at 1400-1800 K have been calculated. The partial pressure, composition of solids and distribution in the gas phase are given, and the effects of oxygen and minor water discussed. The consequence of these results on the formation of large-area, single-crystal Na-beta"-Al2O3 films by solid-vapour reaction is identified.