The synergistic effect of O-2 and SO2 on the corrosion behavior of X70 steel was evaluated in water-saturated supercritical CO2 using weight loss analysis and surface analysis techniques. Corrosion experiments were conducted at 40 degrees C and 100 bar to simulate typical CO2 transportation conditions. Results indicate that the synergistic effects of O-2 and SO2 enhance or inhibit the corrosion rate, depending on the concentration variation of impurities. SO2 had the highest corroding influence on X70 steel, with the corrosion rate of 1.10 mm/year with the addition of 500 ppmv SO2. The presence of SO2 changed the corrosion mechanism and the final products of corrosion consisted mostly of FeSO3. Low concentrations of O-2 increased the corrosion rate (0.09 mm/year) primarily by destroying the integrity of the FeCO3 product film in the absence of SO2. However, the corrosion rate of X70 steel progressively decreased to 0.03 mm/year as the concentration of O-2 increased to 1000 ppmv, due to high concentration of O-2 causing the steel substrate to enter the passivation zone. This study is important for a number of industrial applications, as it provides a theoretical basis for corrosion control of supercritical CO2 pipelines. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.