Results from high-pressure synchrotron x-ray diffraction and high-temperature x-ray diffraction measurements on polycrystalline samples of the tetragonal perovskites Sr2GaSbO6, Sr2NiMoO6, and Sr2FeNbO6 are reported. A phase transition, where the unit cell changes symmetry from tetragonal to cubic, is observed for each compound at elevated temperatures. The phase transition changes the structure from one exhibiting an octahedral tilting distortion at ambient temperature to one that is untilted above the transition temperature. At elevated pressures the  lattice parameter ratio increases, indicating that the magnitude of the octahedral tilting distortion is increasing as a function of pressure. In the pressure range studied, up to ∼6 GPa, no phase transitions were observed.