The Olkaria geothermal system in Kenya is located within the Olkaria volcanic complex in the central sector of the Kenya Rift Valley. Reykjanes, Svarstengi and Nesjavellir geothermal fields are located in southwest Iceland. These are high temperature geothermal systems with temperatures <240°C. Speciation calculations indicate that in Olkaria CO2 partial pressures range between 0.5 bar and 5 bar except in the Olkaria West sector with <90 bars, but lower at Reykjanes, Svartsengi and Nesjavellir. High CO2 partial pressures cause CO2 rich waters to develop when CO2 in steam encounters shallower ground waters. Thermodynamic calculations of HCl content using pH, chloride, and aquifer temperature indicate high content of HCl in the aquifer water of Reykjanes and Svartsengi, but lower at Olkaria and Nesjavellir. Scales formed during tests at Nesjavellir at wells NJ-14 and NJ-22 wellheads consisted mainly of sulphides and mixed sulphides and oxides. In separated water after the heat exchangers, entry to retention tank and at injection well scales consisted mainly of amorphous silica. The highest deposition was at entry to the retention tank. In Olkaria Well OW-34 scales formed in the well