Acidic fluids occur in geothermal well discharges. As they cause serious damage to production wells and transmission pipelines, their origin needs to be understood in order to design appropriate preventive or treatment measures for sustainable geothermal energy production. Realizing the potential contribution of stable isotopes of the water molecule and those of sulphur compounds in tracing the sources of acidic fluids, especially the sulphate type of acidity in geothermal well discharges, the IAEA implemented a Coordinated Research Project (CRP) in which ten research groups carried out field and laboratory investigations on twenty geothermal fields. Scientists in this project used a variety of isotopes as tracers and temperature indicators for geothermal systems. The following major results were obtained: (1) identification of the origin of the water component in acidic fluids, (2) identification of the origin of sulphur compounds in acidic fluids, (3) mixing relations between waters from different sources to form acidic fluids and (4) a test of isotope geothermometry based on sulphur compounds in geothermal fluids. This publication is a compilation of the final reports of the research project with a summary of the findings.