NdBaMnCoO5 has been synthesized using a mass-flow controlled reducing reaction, followed by low (800 degrees C) temperature oxidation to produce NdBaMnCoO6. The structures and properties have been analyzed using powder neutron diffraction and magnetic susceptibility measurements. At room temperature both phases are tetragonal with unit cells a(p) x a(p) x 2a(p) (a(p) = perovskite subcell length) showing full order of the A-site cations and no order of either B-site cations or charge. Low temperature neutron diffraction shows a fully antiferromagnetic (G-type) structure for NdBaMnCoO5 (Cell root 2a(p) X root 2a(p) x root 2a(p)) and an orthorhombic cell (2a(p) x 2a(p) x 2a(p)) for NdBaMnCoO6. (C) 2007 Elsevier Ltd. All rights reserved.