The crystal structure behavior of the Sr2GdRuO6 complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K <= T <= 1273 K. Measurements of X-ray diffraction at room-temperature and Rietveld analysis of the experimental patterns show that this compound crystallizes in a monoclinic perovskite-like structure, which belongs to the P2(1)/n (#14) space group and 1:1 ordered arrangement of Ru5+ and Gd3+ cations over the six-coordinate M sites. Experimental lattice parameters were obtained to be a =5.8103(5) angstrom, b =5.8234(1)angstrom, c =8.2193(9) angstrom, V = 278.11(2) angstrom(3) and angle beta = 90.310(5)degrees. The high-temperature analysis shows the occurrence of two-phase. transitions on this material. First, at 573 K it adopts a monoclinic perovskite-type structure with 12/m (#12) space group with lattice parameters a = 5.8275(6) angstrom, b = 5.8326(3) angstrom, c = 8.2449(2) angstrom, V = 280.31(3) angstrom(3) and angle beta = 90.251(3)degrees. Close to 1273 K it undergoes a complete phase-transition from monoclinic 12/m (#12) to tetragonal 14/m (#87), with lattice parameters a = 5.8726(1) angstrom, c = 8.3051(4) angstrom, V = 286.39(8) angstrom(3) and angle beta = 90.0 degrees. The high-temperature phase transition from monoclinic 12/m (#12) to tetragonal 14/m (#87) is characterized by strongly anisotropic displacements of the anions. (C) 2011 Elsevier Ltd. All rights reserved.