The present study was designed to study the role of impurity adsorption and condensation behaviors in the corrosion mechanism of CO2 transport pipeline steel, based on the hypothesis that the impurity adsorption and condensation play a critical role in inducing the formation of aqueous electrolyte in the initial stage of the corrosion processes. The impurity adsorption and condensation behaviors including water adsorption, adsorption of sulfurous species (SO2/H2SO3), and H2SO3 condensation onto the Fe-coated quartz crystals were measured by the Quartz Crystal Microbalance (QCM) technique in water-unsaturated CO2/H2O flow and CO2/H2O/H2SO3/SO2 flow at 45°C and 1 bar. The connections between the water adsorption and acid condensation behavior onto Fe-coated quartz crystals with Fe corrosion were also studied by utilizing SEM and EDS techniques. The results showed that Fe suffered no corrosion in the water-unsaturated CO2/H2O gas flow but suffered localized corrosion in the water-unsaturated CO2/H2O/H2SO3/SO2 gas flow. It was suggested that the acid condensation initiated the Fe corrosion at water-unsaturated conditions with impurities instead of the pure water adsorption behavior.