Two different types of ab initio electronic and atomic structure calculations for ordered solid solutions are combined with solid solution thermodynamics in a study of the Sr-doped LaMnO3. Unlike Ba in the isostructural (Sr,Ba)TiO3, Sr aggregation in the LaMnO3 matrix potentially leading to decomposition into heterogeneous mixture of LaMnO3 and SrMnO3 phases is energetically unfavorable. We demonstrate that for a particular solid solution with 12.5% Sr the order–disorder transition occurs only at very high temperatures otherwise Sr is supposed to be periodically distributed in the LaMnO3 matrix.