This paper presents measurements of ionic conductivity in single crystals of Li+ and mixed Li+/Na+ beta alumina. The crystals were prepared by ion exchange of Na+ beta alumina crystals in , , or with added. The measured room temperature conductivity of Li+ beta alumina, about , is higher than earlier values. The activation energy was found to be 0.24 eV from −80° to 400°C. Room temperature conductivities of Li+/Na+ beta alumina crystals pass through a minimum as the composition is varied from pure Na+ to pure Li+ beta alumina. The activation energy achieves a maximum at the same composition. The theory of Hendrickson and Bray for mixed‐alkali conduction in glasses is adapted to explain this effect. The deleterious effect of water on lithium transport in beta alumina is also examined.