A thermodynamic approach to water geothermometry Introduction Thermodynamics geochemical modeling and related considerations The selected reservoir liquids The hydrothermal minerals The traditional water geothermometers and fCO2 indicators The activity based theoretical Na K geoindicators The activity based theoretical K Mg and Na Mg geoindicators The activity based theoretical K Ca and Na Ca geoindicators Conclusions and way forwards References. This book explores water geothermometry a highly relevant topic in the exploration and exploitation of geothermal energy. Presenting theoretical geothermometers and indicators of CO2 fugacity it describes a rigorous new approach entirely based on thermodynamics. The book will appeal to geothermal geoscientists especially those working in research institutions and companies around the globe. It is also of interest to students on advanced courses in applied geochemistry water–rock interaction and other related areas.