The lithium ion conducting properties of materials of composition La0.58Li0.26TiO3, Nd0.58Li0.26TiO3, La0.67Li0.25Ti0.75Al0.25O3 and La0.29Li0.12NbO3 have been compared in relation with their microstructure. All the oxides have powder X-ray diffraction patterns characteristic of a perovskite-related structure with lattice parameters a∼√2ap, b∼√2ap, c∼2ap (p refers to cubic perovskite). However, some important differences are observed in their microstructure by SAED and HRTEM. Ordering between vacancies, Li+ and La3+ or Nd3+ and twinning of the NbO6 or TiO6 octahedra tilting system are shown in La0.29Li0.12NbO3 and Nd0.58Li0.26TiO3, which are the materials having a lower ionic conductivity. The La0.58Li0.26TiO3 and La0.67Li0.25Ti0.75Al0.25O3 oxides do not show ordering between cations.