Radionuclides can occur in the environment either through chronic releases of nuclear facilities, or due to incidents or accidents. In order to study there behaviors in the environment (migration, retention, transfer, ...) and in human organisms (metabolism, retention, excretion), it is of prime importance to know their solution chemistry, and more particularly thermodynamic equilibrium constants, allowing to calculate their speciation. In fact, speciation governs the migration, the

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... ity and the toxicity of elements. Moreover, this knowledge is also of great interest for decorporation or decontamination purposes. In this framework, a CEA working group on speciation has been created in order to share data both on thermodynamic constants and on analytical speciation methods, interesting chemists, environmentalists and biologists. It has been focused, in a first time, on actinides and a choice of specific ligands considered to be most representative of environmental and biological media. The thermodynamic database used is BASSIST (developed by CEA) in interaction with the speciation model JCHESS. Different examples are then presented on the selection of data (thermodynamic constants, ligands of interest) through benchmark exercises to test our state of knowledge in this area. Also, analytical methods to determine thermodynamic constants or direct speciation are discussed. Article published by EDP Sciences and available at