The stoichiometry range, crystal chemistry, ionic conductivity, and electrochemical window of the La 2/3 Li x Ti 1-x Al x O 3 system with perovskite-related structure have been studied. The range of the existence of the solid solution is (0.06 ) x ) 0.3). Powder X-ray diffraction and transmission electron microscopy results show that these oxides have a unit cell multiple of the perovskite cell with dimensions a ≈ 2a p , b ≈ 2a p , c ≈ 2a p . The ionic conductivity of the materials and its dependence on composition and temperature have been examined by complex impedance spectroscopy. Lithium ion conductivity increases with increasing lithium content up to a value (7.6 × 10 -5 S cm -1 at room temperature) corresponding to x ) 0.25. Electrochemical experiments indicate that these materials can be used as solid electrolytes in secondary batteries having Li metal as the anode down to 1.6 V.