Precise control of the "one-pot" reaction conditions is employed for the assembly of a nanosized 2.8 nm cluster, [H34W119Se8Fe2O420](54-) (1a), which is templated using eight selenite units and two iron(III) centers. Not only is this cluster characterized structurally using X-ray crystallography, it is the largest polyoxotungstate to be observed intact in solution using ESI-MS, with a molecular mass of ca. 30 000 Da.