A new material of nominal stoichiometry YGaMnO5 has been prepared in polycrystalline form from citrate precursors followed by thermal treatments under high-oxygen pressure. This compound has been characterized from neutron powder diffraction (NPD) data and magnetic measurements. For comparison, the parent compound YMn2O5 has also been synthesized and its crystal structure refined by NPD data. The new oxide has an actual stoichiometry YGa1-xMn1+xO5 (x = 0.23), determined by NPD, showing an important cationic disorder between both metal sites; it is orthorhombic, Pbam (SG), and its crystal structure contains chains of Mn4+O6 edge-sharing octahedra, linked together by Ga3+O5 pyramids and YO8 units. With respect to YMn2O5, containing axially elongated MnO5 pyramids due to the Jahn-Teller effect of Mn3+ cations, the GaO5 pyramidal units in YGa0.77Mn1.23O5 are substantially flattened. This compound has a paramagnetic behaviour with two weak anomalies at about 50 K and 350 K. The magnetic structures, studied at 1.4 K and 100 K show a ferromagnetic coupling along the chains of MnO6 octahedra. (c) 2007 Elsevier Ltd. All rights reserved.