To assess the role of dissolved oxygen in supercritical water on the growth processes of oxide scales, a marker experiment of ferritic-martensitic steel was performed by exposing to supercritical water ((H2O)-O-16) containing marked O-18(2) at 600 degrees C under 25 MPa. The oxide scale was analysed by using Secondary Ion Mass Spectrometry (SIMS). The oxygen isotope profile shows that dissolved oxygen and supercritical water react with metals simultaneously. Dissolved oxygen changes the oxidation/reduction potential of supercritical water leading to an increase in the oxidation rate. The oxidation rate increases with increase in dissolved oxygen content in supercritical water. The mechanism of the effect of dissolved oxygen on oxidation is discussed. (C) 2015 Elsevier B.V. All rights reserved.