Phase stability studies were performed within the quasiternary system LaGaO3-SrGaO2.5-"LaMgO2.5". Emphasis was cast on the temperature dependence of the homogeneity region of La1-xSrxGa1-yMgyO3-delta perovskite solid solutions. Isothermal sections were determined at 1100degrees, 1250degrees, 1400degrees, and 1500degreesC in a static air atmosphere. The single-phase homogeneity region was found to considerably diminish with decreasing temperature, indicating a reduction of the solid solubility of Sr and Mg, and below 1100degreesC the doped perovskite becomes unstable. Consequently, the cubic perovskite phase was found to exist only at elevated temperatures and for high Sr and Mg amounts. Sample preparation was performed by the mixed-oxide process as well as by a modified combustion synthesis.