[en] A thermodynamic database was established for performance assessment of geological disposal of high-level radioactive and TRU wastes based on the thermodynamic database developed by the Japan Nuclear Cycle Development Institute in 1999 (JNC-TDB). Twenty-five elements (actinides, fission products and their daughters) which are important for the performance assessment of geological disposal have been selected. The fundamental plan was in principle based on the guidelines established by the Nuclear Energy Agency in the Organisation for Economic Co-operation and Development (OECD/NEA). Additional unique guidelines were established due to a requirement from the performance assessment to select tentative thermodynamic values obtained from chemical analogues and/or models for elements and species with insufficient thermodynamic values. Selected thermodynamic data will be compiled for geochemical calculation programs. (author)