Corrosive conditions in BWRs are determined mainly by hydrogen peroxide (H{sub 2}O{sub 2}). Then, a high temperature, high-pressure H{sub 2}O{sub 2} water loop was fabricated to identify the effects of H{sub 2}O{sub 2} on corrosion and stress corrosion cracking of stainless steel. By changing concentrations of H{sub 2}O{sub 2} and O{sub 2}, in situ measurements of electrochemical corrosion potential (ECP) and frequency dependent complex impedance (FDCI) of test specimens were carried out and then characteristics of oxide film on the specimens were evaluated by analyzing FDCI data based on the equivalent circuit analysis. The following points were experimentally confirmed. (1) The ECP and FDCI data of the specimens exposed to 100 ppb H{sub 2}O{sub 2} were not affected by co-existing O{sub 2} with the same level oxidant concentration and they were also not affected by pre-exposure to 200ppb O{sub 2}. From the view-point of ECP, this meant that corrosive conditions of hydrogen water chemistry were the same as those of normal water chemistry. (2) The low frequency semi-circles of the FDCI data for the specimens exposed to 100 ppb H{sub 2}O{sub 2} reached a saturation value which was much smaller than saturation values for specimens exposed to 200 ppb O{sub 2} and to 10 ppb H{sub 2}O{sub 2}. (3) Smaller oxide dissolution resistance and larger electric resistance of the oxide film were obtained for the specimens exposed to 100 ppb H{sub 2}O{sub 2}. This caused ECP to increase by shifting the anodic polarization curve of stainless steel to the high potential side. (author)