Knowing the thermo-physical properties of geothermal water reduces the uncertainties in estimating the thermal output of power plants. So far, thermo-physical properties have been investigated under laboratory conditions and for individual geothermal plants, only. In-situ measurements at temperatures above 120° C, at pressures above 20 bar and over a wide salinity-range are sparse. Hence, pure water properties are used to design power plants. PETher project fills this gap: Thermo-physical properties – heat capacity, viscosity, density and thermal conductivity – of geothermal water are measured as functions of temperature, pressure and salinity. Results serve as validation for laboratory measurements and for numerical models.