Solid phase equilibria in the systems La2O3-Ga2O3-MgO and La2O3-Ga2O3-MgO-SrO near the solubility lobe of the (La,Sr)(Ga,Mg)O3-delta perovskite phase were determined by XRD, SEM/EDX and EPMA of 30 samples which were annealed at 1173 and 1673 K and quenched subsequently. Equilibration of the samples at 1673 K was ensured by considering the diffusion coefficients of the cations which are the slowest moving species and the grain size. Solubility limits of Mg or Sr in the perovskite LaGaO3 phase were determined at 1673 K as 14 mol% for Mg and <2 mol% for Sr. The Mg0.9Ga1.1La3O7+delta phase was identified in the La2O3-Ga2O3-MgO and La2O3-Ga2O3-MgO-SrO systems. MgGaLa3O7(s) was, in addition, identified in the La2O3-Ga2O3-MgO system. The solubility of Sr in the La1-xSrxGa1-yMgyO3-((x+y)/2) perovskite phase increases up to about 20 mol% if Mg and Sr are added together. Phase composition of all fields adjacent to the doped perovskite phase in the quasi-ternary section LaGaO3-"LaMgO2.5"-"SrGaO2.5" resulted at 1673 K from experimental data and by comparison with the phase diagrams of the quasi-ternary systems La2O3-Ga2O3-MgO and La2O3-Ga2O3-SrO. The results obtained are compared with the investigations of other investigators. (C) 2004 Elsevier B.V All rights reserved.