Crystal structure and anisotropy of the thermal expansion of single crystals of La1−xSrxGa1−2xMg2xO3−y (x=0.05 and 0.1) were measured in the temperature range 300–1270 K. High-resolution X-ray powder diffraction data obtained by synchrotron experiments have been used to determine the crystal structure and thermal expansion. The room temperature structure of the crystal with x=0.05 was found to be orthorhombic (Imma, Z=4, a=7.79423(3) Å, b=5.49896(2) Å, c=5.53806(2) Å), whereas the symmetry of the x=0.1 crystal is monoclinic (I2/a, Z=4, a=7.82129(5) Å, b=5.54361(3) Å, c=5.51654(4) Å, β=90.040(1)°). The conductivity in two orthogonal directions of the crystals has been studied. Both, the conductivity and the structural data indicate three phase transitions in La0.95Sr0.05Ga0.9Mg0.1O2.92 at 520–570 K (Imma–I2/a), 770 K (I2/a–R3c) and at 870 K (R3c–R-3c), respectively. Two transitions at 770 K (I2/a–R3c) and in the range 870–970 K (R3c–R-3c) occur in La0.9Sr0.1Ga0.8Mg0.2O2.85.