The crystal structure and structural phase transition of La1-xSrxGa1-yMgyO3-delta prepared by heat treatment at 1450 degreesC for 24 h were evaluated by X-ray diffraction measurements, differential scanning calorimetry (DSC) and dilatometry. La1-xSrxGaO3-delta with x larger than 0.05 could not be obtained as a single phase. LaGa1-yMgyO3-delta with 0less than or equal toyless than or equal to0.2 could be prepared as a single phase, although Bravais lattice varied from primitive orthorhombic to a body-centered one with substitution of y=0.1 Mg. Partial substitution of Sr for La site in LaGa1-yMgyO3-delta whose y=0.15 and y=0.20 was possible with solution limit of 0.10 and 0.20, respectively. The crystal system changed from orthorhombic to cubic at Sr content of 0.15 in La1-xSrxGa0.8Mg0.2O3-delta. The structural phase transition to rhombohedrally distorted perovskite by temperature was observed in orthorhombic La1-xSrxGa1-yMgyO3-delta, whose transition temperature increased with increase of Sr and Mg content. DeltaH and DeltaV at the phase transition drastically decreased with increase of Mg content, which could be ascribed to smaller distortion from ideal perovskite in body-centered orthorhombic than in primitive orthorhombic. (C) 2004 Elsevier B.V. All rights reserved.