The metastable crystal structure of strontium- and magnesium-substituted LaGaO3 (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm) to rhombohedral (space group R(3) over bar c) at the composition La0.825Sr0.175Ga0.825Mg0.175O2.825 and, eventually, to cubic (space group Pm3m) at the composition La0.8Sr0.2Ga0.8Mg0.2O2.8. At 500degreesC in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm) to cubic (space group Pm (3) over barm) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be alpha(a,ortho) = 10.81 X 10(-6) K-1, alpha(b,ortho) = 9.77 x 10(-6) K-1, and alpha(c,ortho) = 9.83 X 10(-6) K-1 (25degrees-400degreesC), and for the cubic modification to be alpha(cubic) = 13.67 x 10(-6) K-1 (500degrees-1000degreesC).