Multiple inhibitors are commercially available to mitigate the formation of both scaling and corrosion precipitates. However, the conditions in a geothermal installation are often extreme. This includes high temperatures and high pressures besides the presence of a possibly very saline fluid. In the Balmatt geothermal installation of Mol (Belgium) pressure was kept at 45 bar, which is higher than most other geothermal installations. The temperature of the brine in the installation is ∼120 °C before it passes through the heat exchanger. The amount of totally dissolved solids (TDS) in the Balmatt brine is 165 g L-1 and it has a high ionic strength (∼3.5 mol L-1). To evaluate if inhibitors could have possibly caused the precipitation of the amorphous organic phase observed in the filter scales, a series of experiments was performed. Two inhibitors (DETPMP (diethylenetriamine pentamethylene phosphonic acid) and TD1100) were selected for high pressure and high temperature (HP-HT) experimental tests using autoclaves. Both inhibitors were exposed to conditions present in the surface installation of the Balmatt site, i.e. high temperatures (75 °C and 120 °C) and a high pressure of 45 bar to evaluate their stability. They were mixed with synthetic brine and pressurized and heated in separate autoclaves. Both inhibitors showed poor stability at the HP-HT conditions of the surface installation of the Balmatt site. Precipitates on the borders of the Teflon insert pot were observed (mainly iron oxides). In the synthetic brine - DETPMP mixture, additional precipitates besides the precipitation on the borders of the Teflon insert pots, were observed after being exposed to high temperatures and high pressure. The precipitates consisted of an amorphous matrix containing CaCl2, Fe-oxides and some minor Ca-rich BaCl2 minerals. The DETPMP and TD1100 inhibitors are not stable at the conditions present in the Balmatt geothermal installation. In order to investigate the efficiency of inhibitors to prevent the formation of galena scaling in the Balmatt geothermal installation, laboratory experiments with synthesized galena, three different background solutions (synthetic brine, NaCl solution and milliQ water) and three inhibitors were performed. According to the knowledge of the authors, no specific inhibitors to prevent galena formation are commercially available. Based on literature research, the DETPMP, TD1100 and ScaleTrol inhibitors were selected for this research. The DETPMP prevented the formation of galena in synthetic brine but other Pb-rich phases were formed. The TD1100 and ScaleTrol inhibitors did not prevent the formation of galena in synthetic brine. In the NaCl solution, all three inhibitors seemed to work well for galena inhibition. However, other Pb-rich phases were formed. No formation of galena was observed in the milliQ water (with or without inhibitors).