The crystal structures of Ca0.6Sr0.4TiO3 and Ca0.8Sr0.2TiO3 perovskites have been studied at high temperatures by X-ray powder diffraction. The most likely sequence of phase transitions with increasing temperature in Ca0.6Sr0.4TiO3 is the same as that shown by the system CaTiO3-SrTiO3 with increasing Sr content at room temperature. This sequence is: orthorhombic Pbnm, orthorhombic Bmmb, tetragonal I4/mcm and cubic Pm3̄m. The second orthorhombic phase could not be determined in Ca0.8Sr0.2TiO3, either due to the poorer resolution of the diffractometer employed or because the Bmmb phase might not have a stability field at this composition. The transition temperatures decrease on going from Ca0.8Sr0.2TiO3 to Ca0.6Sr0.4TiO3. Calorimetric measurements on 12 samples spanning the system CaTiO3-SrTiO3 then allow one to draw, in combination with the RT XRD data, the temperature vs. composition phase diagram. Phase boundaries show a linear shift toward lower Sr content with the increase of temperature, implying a negligible plateau effect near the CaTiO3 composition, but quantum saturation occurs at low temperatures near the SrTiO3 composition.