The structure of the oxygen sub-lattice in Cs 0.5 [Nb 2.5 W 2.5 O 14 ] is investigated for the first time by advanced electron-optical methods. Since Cs 0.5 [Nb 2.5 W 2.5 O 14 ] resembles the crystal structure of the so-called M1 phase of Mo-V-Nb-Te-O, which is the best catalyst for the selective oxidation of propane to acrylic acid, the structure analysis of the oxygen sub-lattice can contribute substantially to a better understanding of this catalytic process. The so far only partially known structure of the complex metal oxide Cs 0.5 [Nb 2.5 W 2.5 O 14 ] is investigated by combining modern methods of high-resolution transmission electron microscopy with crystallographic image processing techniques. Average atom positions in the a-b plane are determined from the phase of the exit-plane wave function, which was numerically reconstructed from a focal series of spherical-aberration-corrected electron micrographs. The experimentally determined atom positions agree well with a structure model obtained by first-principles calculations for the underlying M 5 O 14 (M = Nb, W) framework. Moreover, several deviations from the periodic structure, which may influence the catalytic properties of the structurally similar M1 phase, are observed. The obtained results demonstrate that aberration-corrected high-resolution transmission electron microscopy is a reliable alternative for the analysis of periodic structures, in particular when traditional methods for crystallographic structure determination like XRD or neutron diffraction cannot be applied.