The efficiency of geothermal utilisation depends on the behaviour of fluids that transfer heat between the geosphere and the engineered components of a power plant. The EU-funded REFLECT project aims to avoid problems related to fluid chemistry rather than treat them. The physical and chemical fluid properties are often poorly defined, as in situ sampling and measurements at extreme conditions are difficult to date. Therefore, large uncertainties in current model predictions prevail, which will be tackled in REFLECT by collecting new, high-quality data in critical areas. These data will be implemented in a European geothermal fluid atlas and in predictive models allowing to provide recommendations on how to best operate geothermal systems for sustainable use.