The immersion experiment was carried out to study the corrosion behavior of low-alloy steel with martensite/ferrite microstructure at vapor-saturated CO2 and CO2-saturated brine conditions. The characteristics of corrosion behavior are investigated by the microstructure of tested steel, corrosion rate, corrosion phase, surface morphology and cross-section morphology. The results demonstrate that the microstructure of tested steel consists of tempered martensite and ferrite, and ferrite may dissolve preferentially in corrosion process. The corrosion products comprising of inner layer and outer layer at both vapor-saturated CO2 and CO2-saturated brine conditions are Cr-rich compound and FeCO3, respectively. The corrosion rate at vapor-saturated CO2 condition is tremendously lower than that at CO2-saturated brine condition. The corrosion rate and scale morphology show that the compact and dense outer layer plays an important role on corrosion resistance, although the inner layer appears firstly. Based on the results of this experiment, the underlying mechanisms of low-alloy steel with martensite/ferrite microstructure are proposed to illustrate carbon dioxide corrosion behavior. (C) 2015 Elsevier B.V. All rights reserved.