- We suggest theoretical prediction for BacSr1−cTiO3 perovskite solid solutions (BST) combining ab initio DFT/B3PW calculations and alloy thermodynamics. This approach is based on calculations of a series of ordered super-structures in Ba–Sr simple cubic sublattice immersed in the rest TiO3 matrix. Although these structures are unstable with respect to the decomposition, the results of total energy calculations allow us to extract the necessary energy parameters and to calculate the phase diagram for the solid solutions (alloys). A novel approach applied to the BST system enables to predict that at T > 400 K Ba and Sr atom distribution is random. But below this temperature at small c Ba atoms aggregate into nanoclusters, thus leading to the formation of Ba-rich complexes of “almost pure” BaTiO3 (BTO) in mostly SrTiO3 (STO) matrix. At large c the formation of analogous SrTiO3 complexes in BaTiO3 is predicted.