Decommissioning data collection has been, up to now, almost a national matter. The present joint study performed by NIS Ingenieurgesellschaft mbH, Hanau, and the Commissariat a l`energie atomique, Unite de declassement d`installations nucleaires, and coordinated by the Commission of the European Communities, is intended to identify a methodology allowing the collection of data which are useful to manage a decommissioning project, e.g. data on dismantling costs, occupational radiation exposure and waste arisings. A common structure for decommissioning tasks (called working packages) derived from those already in use at the abovementioned organizations has been established and the corresponding first series of data-collection sheets defined in order to cover data from LWRs, GCRs (UNGGs) and nuclear fuel cycle installations. Work focused on the cost of decommissioning tasks. The study is intended to be a first step towards an EC-wide usable data-base for data generated in the various ongoing and future decommissioning projects.