The evolution of the room-temperature structure of the oxide series La_2−xSr_xCoTiO₆(0.6 ≤x≤ 1.0) was studied as a function of the Sr content using different diffraction techniques and applying the symmetry-adapted modes formalism (AMPLIMODES). The title compounds adopt perovskite-like structures of rhombohedral symmetry with an octahedral tilting scheme (a⁻a⁻a⁻) with either space group R\overline 3c orR32. The latter symmetry is observed in those cases (forx≃ 0.6) where additional rock-salt-like ordering of La and Sr is produced in the perovskiteAsites. Two composition-driven phase transitions are observed in the whole series La_2−xSr_xCoTiO₆(0.0 ≤x≤ 1.0). Using the concept of internal pressure, the effect of doping with Sr on the structure can be properly discussed. Both phase transitions seem to be of first order since they can be associated with discontinuities either in the entropy or in the structure. The first transition (P2₁/n→Pnma) occurs as theBcations become totally disordered. Along the whole compositional range the modes responsible for the out-of-phase tilting ofBO₆octahedra remain active, but those associated with the in-phase octahedral tilting vanish forx≥ 0.6, this being associated with the second transition (Pnma→ R\overline 3c). Finally, forx= 1.0 the three pseudo-cubic cell parameters become very similar, pointing to a transition to a cubic structure which could be obtained by applying pressure or raising the temperature.