The non‐stoichiometry of the perovskite‐type phase Sr1—αxLaxTiO3 + x(1.5—α) is studied. Pure compounds with 0.9 ⪅ α ⪅ 1.5 could be obtained. This non‐stoichiometry can be ascribed to a distribution of La(III) ions and vacancies over Sr and Ti sites, which varies with composition. A tetragonal superstructure due to an ordering of defects in layers, is observed for a compound with x = 0.50 and α = 1.50. Dielectric relaxations with 0.3 ⪅ Eact ⪅ 0.4 eV and τ0 of the order of 10−12 s were observed for various compositions. These relaxations are characterized by a narrow distribution of relaxation times and are due to an ionic polarization mechanism.