Hydrazine is added into secondary circuit in order to eliminate oxygen and to impose a reducing potential. Another possible role of hydrazine is to reduce hematite potentially present and thus reduce risk of stress corrosion cracking of steam generator tubes. However, the latter effect of hydrazine is still not clear. In order to better understand the mechanism and the kinetics of hematite reduction by hydrazine under secondary circuit conditions (275°C, slightly alkaline, diluted hydrazine), an experimental study has been carried out. It was found that the reduction of hematite proceeds through the adsorption of hydrazine, surficial reduction and dissolution of hematite, and coprecipitation of well crystallized octahedral magnetite.