The new layered oxides Ca 2 MnGaO 5ϩδ and Sr 2 MnGaO 5ϩδ were synthesized by solid state reaction from stoichiometric amounts of CaO (SrO), Mn 2 O 3 and Ga 2 O 3 in evacuated silica tubes and annealed at 1000 °C for 8 days with one intermediate regrinding. After annealing the samples were quenched. The crystal structures of Ca 2 MnGaO 5.045 and Sr 2 MnGaO 4.97 were studied using X-ray, neutron and electron diffraction and HREM. The samples of Ca 2 MnGaO 5.045 consist of two closely intermixed brownmillerite-type phases with Pnma or Ima2 space symmetry. They possess closely related structures that differ mainly by the orientation of the Ga-O chains. 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 HREM showed that local ordering of the left-hand and right-hand chains of GaO 4 tetrahedra in Sr 2 MnGaO 4.97 leads to a super-structure with a doubling of the b parameter of the unit cell. The formal Mn valence (V Mn ) in Ca 2 MnGaO 5ϩδ can be varied by oxygen insertion. The treatment under 80 atm O 2 results in the Ca 2 MnGaO 5.39 phase built of domains of initial reduced material and fully oxidized Ca 2 MnGaO 5.5 phase. The increase of V Mn from ϩ3.09 to ϩ3.78 is accompanied by a drastic compression of the structure along the b ϭ 4a per axis due to a suppression of the Jahn-Teller deformation of the MnO 6 octahedra. The formal oxidation state of Mn (V Mn ) can be varied by thermal treatments at 415 °C under the required oxygen partial pressure or at elevated oxygen pressure (450 °C, 20 atm O 2 ). The samples belong to two regions: one of them embraces oxidation states of Mn of ϩ2.94 Յ V Mn Յ ϩ3.26 (a low-oxidized region) and the second one ϩ3.82 Յ V Mn Յ ϩ4.01 (a highly-oxidized region). The syntheses in O 2 -filled silica tubes revealed that the sample with an average V Mn ϭ ϩ3.4 consists of a mixture of two phases, which belong to the low-oxidized and highly-oxidized regions, respectively. The Xray (XRD) powder patterns of the samples from the low-oxidized region were indexed on an orthorhombic brownmillerite-type unit cell with a ഠ c ഠ a p √2 and b ഠ 4a p . The following oxygen insertion lead to the formation of a tetragonal perovskite-like structure Sr 2 MnGaO 5.47 (a a p , c 2c p ) with oxygen vacancies located in the Ga-layers. The observed variation of unit cell parameters with oxygen non-stoichiometry can be explained by a decrease of the average size of Mn cations on going from a oxidation state ϩ3 to ϩ4, a suppression of Jahn-Teller distortion of MnO 6 octahedra due to a gradual transition from t 2g 3 e g 1 to t 2g 3 e g 0 electronic configuration.