The Use of Thermodynamic Databases in Performance Assessment : Workshop Proceedings Barcelona Spain 29 30 May 2001 Performance assessment of repository concepts for the geological disposal of long lived radioactive waste relies on the availability of thermodynamic data for many radionuclides and other elements under a wide range of physico chemical conditions. For the past ten years the OECD Nuclear Energy Agency (NEA) has been co ordinating a multinational effort to produce a database of selected thermochemical values that would satisfy the requirements of the various national programmes in Member countries. This project is known as the NEA Thermochemical Database (TDB) Project. This publication contains the full papers and summary discussion records of a workshop attended by scientists active in the field of chemical thermodynamics and experts in repository performance assessment who use the thermochemical databases for their evaluations. During the workshop participants discussed current experimental and theoretical standpoints new data requirements and the peculiarities of their application in performance assessment.