Two simple and robust designs for an external high-temperature reference electrode are described. The first external reference electrode uses a movable polytetrafluoroethylene (PTFE) piston to maintain the pressure balance between the reference solution and test solution. The second external reference electrode uses the test solution as reference solution, and this is feasible if the test solution contains a minimum amount of chloride. For both electrodes it is necessary to determine the potential drop across the (cooled) salt bridge, which connects the reference electrode to the autoclave. It is shown that a simple correction algorithm can be used to calculate this potential drop and that the corrections are in agreement with literature values. It is also shown that this correction algorithm is independent of the solution composition for a wide range of neutral and alkaline solutions. The potential difference between two standard hydrogen electrodes (SHE) at different temperatures has been determined. This potential difference has been used to extend the correction algorithm to be able to calculate the potential against the SHE at 25degreesC.