The Nagra/PSI Chemical Thermodynamic Data Base 01/01 is an encyclopedia of thermodynamic data recommended for environmental studies. The data base focuses on elements commonly found as major solutes in natural waters, and on actinides and fission products relevant for radioactive waste disposal projects. It is the official chemical thermodynamic data base used in Swiss radioactive waste disposal projects. The detailed discussion of every number recommended in this encyclopedia is the result of a multi man-year project of the Paul Scherrer Institut (PSI), a Swiss National Lab. The five authors of this work have many years of experience in research, data base development and the application of thermodynamic data in environmental studies. The data included for many elements are based on their reviews of the basic literature. In addition, the data base includes additional data selected by the authors from recommendations of other experts in ground water geochemistry and of the international data base project of the Nuclear Energy Agency (NEA).