The crystal structure of the defect perovskite Ce1/3NbO3, cerium niobium oxide, has been re-examined by neutron powder and electron diffraction. The results of a powder neutron Rietveld refinement indicate that the structure is monoclinic: space group
P2/ mwith Z= 4, a= 5.5267 (3), b= 7.8824 (2), c= 5.5245 (3) Å, β = 90.294 (1)°, V= 240.67 (2) Å3 at 298 K with χ2 = 2.570. Previous reports have described the Ce1/3NbO3 structure in a smaller ( V/2) orthorhombic cell based solely upon X-ray powder diffraction data. The presence of weak reflections in the electron diffraction pattern provides conclusive evidence for a monoclinic superstructure of the orthorhombic cell. While these superlattice reflections are barely detectable with X-ray radiation, they are clearly visible in the neutron diffraction experiments. The superlattice reflections are shown to arise from a tilting of the NbO6 octahedra which results in the reduction of symmetry from orthorhombic to monoclinic. It is also found that the Ce3+ and Nb5+ cations are displaced from the centres of their respective polyhedra to accommodate the bond-valence requirements of the crystal structure. It is likely that distortions of this type are present in other Ln1/3NbO3 and Ln1/3TaO3 defect perovskites.