Current research on cladding and in-core materials for the supercritical water reactor (SCWR) concept is focused on highly alloyed austenitic Fe-Cr-Ni alloys. Standard measurements of the general corrosion rate by exposure testing in autoclaves provide uncertain reproducibility, especially when comparing results from different experimental facilities. Using in-situ electrochemical methods to measure the corrosion rate may be less affected by typical issues, such as exfoliation or deposition, although differences between facilities may still be expected. The use of electrochemical methods is, however, more challenging due to complicated instrumentation as well as due to the high operating conditions. This work is focused on in-situ electrochemical impedance spectroscopy (EIS) measurements of corrosion rate of Alloy 800H at 25 MPa, and temperatures of 380 and 500 °C in deoxygenated and mildly oxygenated (150 µg∙kg-1 of dissolved oxygen) demineralized water. The kinetic curves are presented, and the obtained integral corrosion rates are listed. Significant differences were observed in the concentration profiles of alloying elements between the oxide layers formed at 380 and 500 °C.