The effect of high-temperature treatments on the structure and lithium mobility of the Li_3xLa_2/3-xTiO₃ (0.03 ≤ x ≤ 0.167) series has been studied by HTXRD, Raman, NMR, and impedance spectroscopies. Room temperature XRD patterns of slowly cooled samples display a c-doubled perovskite with orthorhombic or tetragonal symmetries, whereas those of quenched samples display a simple cubic perovskite. However, Raman spectra of analyzed samples are interpreted, in all the cases, with a tetragonal symmetry in which cation disorder increases with the lithium content and quenching treatments. The existence of small microtwinned domains, arranged along the three directions of the perovskite, favors detection of the cubic phase in XRD patterns. From ⁷Li NMR spectroscopy a two-dimensional Li motion was detected in ordered samples, which becomes progressively three-dimensional as cation disordering increases. The presence of microdomains decreases the dc-conductivity of quenched Li-poor samples.