In geothermal energy production, precipitation/scaling and potential plugging of wells is one of the major threats to flow assurance. Mitigation strategies should be considered at the design stage, but to do so knowledge about solubility across a relevant space of temperature, pressure and fluid composition is needed. In hot fluids, silica scaling is often found to be the major concern. Predictive models available in literature give good agreement with experimental values in the region where data are available, but there is limited data available at high temperatures and low pressures, in particular in the presence of salt. This work describes an experimental setup designed to carry out solubility experiments up to 500 °C and 400 bar. A main feature of the setup is the ability to dilute the sample fluid in the hot zone in order to avoid precipitation during sampling. Illustrations are given for how existing phase relations can be used to guide the execution of such high-pressure high-temperature experiments. Results of silica solubility experiments in the presence of NaCl up to 440 °C at 150-350 bar are presented. This work includes data at the steam side pseudocritical line (critical density isochor) where limited data is available in the literature. The experimental concept, with hot zone dilution to avoid precipitation during sampling, can be used to obtain fluid samples as a function of time. Although utilized in the current work for solubility experiments it should be equally applicable to corrosion studies.