High-temperature X-ray powder diffraction experiments have been performed to investigate precisely the lattice parameters of an ionic conducting material La0.683Ti0.95Al0.05O3 perovskite. This material was orthorhombic (space group P/mmm) at 29 and 192 degrees C, while it was tetragonal (space group P4/mmm) from 392 to 1402 degrees C. The orthorhombic-tetragonal phase transition is a possible explanation for the change in the temperature dependence of ionic conductivity around 400 degrees C.