Phase equilibria in the system BaO-SrO-SiO2 in air were established using conventional solid-state techniques. The phase relations in the boundary system SrO-SiO2 were completed by determining the liquidus temperatures for the SrO-rich portion of the diagram. The known compounds, SrSiO3, Sr2SiO4, and Sr3SiO3, melted congruently at 1580°±15°, 2325°±15°, and 2170°±15°C, respectively. Simple binary eutectics exist between (1) Sr2SiO4 and Sr3SiO5 at 2150°±15°C and ∼27 mol% SiO2 and (2) Sr3SiO5, and SrO at 2080°±15°C and ∼23 mol% SiO2. All Alkemade lines were established for the ternary system; two of these joins were examined in detail because of extensive solid-solution regions. On the Ba2SiO4-Sr2SiO4 join, a maximum solubility of ∼70 mol% Sr2SiO4 in Ba2SiO4 appears to exist at 2100°±15°C, the eutectic reaction temperature. A two-phase region apparently connects the eutectic reaction isotherm with a high-temperature polymorphic transition (α′±α) of Sr2SiO4. Below this transition temperature, a complete solid-solution series exists between Ba2SiO4 and α′-Sr2SiO4. The BaSiO3-SrSiO3, join contains limited solid-solution regions at the extremities which arise from the solid solubility of 40 mol% SrSiO3 in the high-temperature (β) polymorph of BaSiOs3 and 20 mol% BaSiO3 in SrSiO3 at 1210°±15°C. The only ternary compound isolated, BaSrSi3O8, has a lower limit of stability at 1220°±3°C and melts incongruently at 1275°±3°C.