A new phase, Bi 2 Nd 4 O 9 , was synthesized in the binary system Bi 2 O 3 –Nd 2 O 3 and structurally characterized from powder X-ray diffraction data. It crystallizes in monoclinic symmetry with lattice parameters a=6.7037(2) Å, b=3.9062(1) Å, c=3.9542(1) Å, β=125.258(1)°, Z=2, and a subcell space group of C2/m. Its average structure had been determined by X-ray powder diffraction and refined by the Rietveld method. The average structure can be described as an oxygen-deficient fluorite structure of the δ-Bi 2 O 3 type with Nd and Bi randomly distributed on the cation site, similar to Bi 2 La 4 O 9 reported by Chen et al. (J. Solid State Chem. 124, 300 (1996)). In addition, a strong superstructure had been observed by transmission electron microscopy.