The crystal structure of the columbite-type phase MgNb 2 O 6 has been refined from powder neutron diffraction data. The compound is orthorhombic, space groupPbcn(60),Z=4, with unit cell parametersa=14.1875(1),b=5.7001(1),c=5.0331(1) Å. The structure contains chains of NbO 6 octahedra sharing edges along thecaxis, which are arranged in double layers through common corners. The double layers, parallel to thebcplane, are connected via MgO 6 octahedra sharing corners. This compound contains Nb 5+ with electronic configurationd 0 . Attempts to obtain new materials with Nb in a mixed valence state were made in the ternary system MgO–Nb 2 O 5 –NbO. For molar ratios Mg:Nb=1:2 new pseudobrookite-type phases have been identified. They have the general composition Mg 5−x Nb 4+x O 15−δ , withxvalues ranging from 1.14 to 1.60. The compounds Mg 4 Nb 2 O 9−δ (corundum-type) and Mg 3 Nb 6 O 11 were identified when the Mg:Nb ratio was 4:2. All the reduced materials were also characterized by thermogravimetric analysis.