The High-Temperature Fluid Measuring Cell is a flow-through device that allows to measure the electrical conductivity of highly corrosive aqueous fluids in dependence of pressure and temperature. So far, the facility was used to study geothermal high-temperature reservoirs at temperatures up to 425°C and fluid pressures up to 350 bar. Beyond the determination of intrinsic fluid properties, the measuring cell is capable to study the effect of fluid–rock interactions at supercritical conditions. As most steels are not corrosion-resist under long-term exposition of supercritical water, the reactor tube was lined with a pipe of corundum ceramic into which two pairs of current and potential electrodes (Pt) as well as a thermocouple (Pt/PtRh) are sealed. The electrode spacing is variable between 25 and 130 mm, what allows the additional integration of solid material into the measuring cell for reactive flow experiments. Fluid samples can be taken down-stream for chemical analysis.