Abstract The crystal structure of sodium tantalum oxyfluoride, β-Na 2Ta 2O 5F 2 has been determined from single crystal diffraction data. The symmetry is monoclinic, space group C2 m with a cell having dimensions: a = 12.855 ± 0.008, b = 7.349 ± 0.005, c = 12.833 ± 0.003 Å, β = 108.97 ± 0.05° and containing eight formula units. The structure was refined by full-matrix least-squares to a final R value of 0.072. The structure consists of two interpenetrating sublattices: the first, which has the overall formula Ta 16X 52, is made up of TaX 6 octahedra and the second with an overall formula Na 14X 4 is composed of Na 4X tetrahedra. The two remaining sodium atoms occupy the center of a hexagonal bipyramid. This arrangement can be described as a succession of weberite and pyrochlore type slabs parallel to the  direction.