The MetroFission project, a Joint Research Project within the European Metrology Research Program, aims at addressing a number of metrological problems involved in the design of proposed Generation IV nuclear reactors. As part of this project a secondary neutron fluence standard is being developed and tested at the neutron time-of-flight facility GELINA of the JRCs Institute for Reference Materials and Measurements. Such a secondary standard will help to arrive at the neutron cross section measurement uncertainties required for the design and safety assessment of new generation power plants and fuel cycles. Such a neutron fluence device contains targets for which the neutron induced cross section is considered to be a standard. A careful preparation and characterization of these samples is an essential part of the development of the secondary standard. In this framework a set of sup.235U targets has been produced by vapour deposition of UF.sub.4 on aluminium backings by IRMMs target preparation laboratory. These targets have been characterized for both their total mass and mass distribution over the sample area. Keywords sup.235U * Characterization * Mass determination * Alpha-particle counting * Metrology