As aqueous processing moves to higher temperatures and pressures to take advantage of increased kinetics there is a need to develop and test appropriate reactor materials to ensure that corrosion is minimized. Corrosion testing often requires an electrochemical approach to fully understand the range of behaviours which should be expected from a corroding metal or alloy. Prior art of designs for electrodes, associated pressure vessels and sealing technology is presented. The development of an apparatus and methods for high temperature and high pressure electrochemical corrosion testing are discussed. The final electrochemical cell design which was developed for temperatures and pressures in excess of 500°C and 5000 PSI is presented.