New Sr 2 MnGaO 4.97 complex oxide was synthesized by solid state reaction in sealed silica tubes at 950–1000°C. The Sr 2 MnGaO 4.97 crystal structure was refined from X-ray powder diffraction data. Sr 2 MnGaO 4.97 is based on the Ima2 brownmillerite-type structure with apically elongated MnO 6 octahedra due to a Jahn–Teller effect. Electron diffraction and high-resolution electron microscopy showed that local ordering of the left-and right-hand chains of GaO 4 tetrahedra in Sr 2 MnGaO 4.97 leads to a superstructure with a doubling of the b parameter of the orthorhombic unit cell. The formal oxidation state of Mn (V Mn ) can be varied by thermal treatments at elevated oxygen pressure (450°C, 20 bar of O 2 ). The oxygen insertion induces a structure transformation in oxidized Sr 2 MnGaO 5.47 material with the formation of a tetragonal perovskite-like structure (a≃a p , c≃2c p ) with oxygen vacancies located in the Ga layers. The oxidation is accompanied by a significant compression of the Mn–O apical distances and a suppression of the Jahn–Teller deformation. Both Sr 2 MnGaO 4.97 and Sr 2 MnGaO 5.47 can probably be treated as canted antiferromagnets with T N =150 and 80 K, respectively.