Single crystals of a new compound, Fe 6.5V 11.5O 35, have been prepared by hydrothermal synthesis at 650°C and 2 kbar. The compound has triclinic symmetry, P overline1, Z = 1 , with unit cell dimensions a = 10.209(3), b = 9.387(3), c = 6.564(2) Å, α = 100.52(5), β = 94.35(5), γ = 98.85(4)°. Its structure was determined using direct methods and refined to an R-value of 0.053 ( wR = 0.038) for 5654 observed reflections ( F > 3 σ( F)) whose intensities were measured on a Philips PW1100 diffractometer using Ag Kα radiation. The structure is based on a cubic close-packed framework of anions in which cations are ordered into tetrahedral (V 5+), square pyramidal (V 4+), and octahedral (Fe 3+, V 4+,3+) sites. The structure comprises zig-zag chains of eight edge-shared octahedra (α-PbO 2-like) which are cross linked via corner-sharing with tetrahedra and square pyramids, and via edge-sharing with square pyramids and octahedra. Bond length-bond strength considerations were used to establish the cation valence-state assignments at the different sites, giving a unit cell composition Fe 3+6.5V 3+1.5V 4+4V 5+6O 35.