Synopsis -A monoclinic superstructure occurring in a Ce 10 W 22 O 81 compound was identified by transmission electron microscopy. The partial oxidation of Ce 3+ along with interstitial oxygen ions could explain this superstructure.Abstract -A complex rare earth tungstate structure, present in a two-phased powder, was solved by electron diffraction, precession and high resolution electron microscopy. The orthorhombic space group Pbnm and the atomic positions deduced from X-rays diffraction experiments were confirmed for Ce 10 W 22 O 81 . A C2/c monoclinic superstructure, with cell parameters a = 7.8 Å, b = 36.1 Å, c = 22.2 Å and β = 100.2°, was evidenced and attributed to a partial oxidation of Ce 3+ leading to interstitial oxygen ions.