The anion-excess ordered fluorite-related phase Ba2.1Bi0.9(O, F)(6.8-delta) has been synthesized by a solid state reaction of BaF2, BiF3 and Bi2O3 at 873 K with subsequent short annealing at 573 K. The crystal structure of the new phase has been solved using electron diffraction and X-ray powder diffraction (a = 9.5372(1) angstrom, c = 18.1623(3) angstrom, space group I4/m, R-I = 0.025, R-P = 0.029). Interstitial anions in the fluorite-based structure are considered to form isolated cuboctahedral 8:12:0 clusters. The structural relationship between the oxyfluoride phase Ba2.1Bi0.9(O, F)(6.8-delta) and similar rare-earth-based fluorides is discussed. (c) 2006 Elsevier Ltd. All rights reserved.