Lanthanum rhenium oxide, La 3ReO 8, crystallizes in the monoclinic system, space group C2 1 with the unit-cell dimensions a = 17.535(11), b = 11.889(7), c = 12.816(8) Å, γ = 90°, Z = 16. Single-crystal diffraction data were collected on an automatic four-circle diffractometer. The structure, determined by Patterson and Fourier methods, has been refined by full-matrix least-squares calculations to an R value of 0.09 for 4206 independent observed reflections. The presence of two superstructures along the a and b axes has made the determination of positions of oxygen atoms very difficult. It can be visualized as isolated ReO 6 octahedra defining tunnels, parallel to the b axis, occupied by a double chain of edge-shared La 4O tetrahedra.