Corrosive conditions in primary cooling systems of boiling water reactors are determined by oxidants, e.g., O2, H2O2 and other corrosive radiolytic species. A high temperature high pressure water loop, which controls H2O2 concentration with minimal O2 co-existence, was constructed in order to evaluate the effects of H2O2 on corrosive conditions, and then static and dynamic responses of stainless steel specimens exposed to H2O2 and O2 in high temperature pure water were evaluated by analyzing ECP (electrochemical corrosion potential) and FDCI (frequency dependent complex impedance). At the same time, in- situ measurements were carried out to determine changes in oxide film electric resistance. From the multilateral high temperature measurements, it was concluded that H2O2 concentrations could be determined by in- situ measurement based on a sensor complex for ECP and FDCI and that ECP and [H2O2] data could be used to evaluate occurrence and propagation of intergranular corrosion cracking of reactor internal components.