[en] Single crystals of lithium--sodium beta alumina with 40 to 95% substitution of Li⁺ for Na⁺ were annealed in air and in vacuum at temperatures up to 1000⁰C. The specimens were examined before and after each heat treatment by pulsed NMR, Raman scattering, infrared absorption, and x-ray diffraction. After annealing at 600⁰C or above, the two original Raman bands due to ⁷Li⁺ vibrations merged into a single peak accompanied by a narrowing of spinel block phonon bands. The 1000⁰C annealing introduced a broad ⁷Li NMR response and substantially reduced narrow line ⁷Li intensity. X-ray difffraction patterns showed that a second phase grows epitaxially with the beta-alumina structure as a result of high temperature heat treatment. The present results suggest that the thermal induced changes observed are due to diffusion of lithium out of the conducting plane and into the spinel-block