Previous high temperature studies have shown that, in an oxygen-free aqueous H2S environment, magnetite (Fe3O4) forms as an inner layer while iron sulfides are found in the outer layer. Although magnetite is thermodynamically less stable than iron sulfide, it was always observed as a defined inner layer. In this work, experiments were conducted to investigate the formation mechanisms of magnetite and iron sulfide in an H2S environment at high temperature. The corrosion behavior of mild steel was first investigated in environments with and without H2S at pH 4.0 and 120 degrees C, showing that magnetite is the dominant corrosion product layer in the initial stage of corrosion, due to a much higher saturation value than iron sulfide (mackinawite). In another experiment, the conversion of magnetite into mackinawite was investigated by exposing a preformed magnetite layer on an inert metal (nickel) to an H2S environment. Consequently, it is postulated that Fe3O4 experiences a simultaneous and continuous process of formation at the steel/magnetite interface and conversion to mackinawite at the magnetite/mackinawite interface. A descriptive model for the formation mechanisms of magnetite and iron sulfide at high temperature is presented. (C) 2018 The Electrochemical Society.