A large number of CO2 dependent chemical, electrochemical and mass transport processes occur simultaneously on and close to the corroding steel surface. The various reactions respond differently to changes in CO2 partial pressure, temperature, water chemistry, flow and other operational parameters. Apparently contradictory observations and results have been reported when these reactions have been studied. The present paper discusses the current understanding of these fundamental CO2 corrosion mechanisms with particularly focus on how CO2 affects the water chemistry and the electrochemical reactions on the bare steel surface. As many sweet systems contain organic acids that affect the solution chemistry and the formation and stability of the FeCO3 corrosion product films, some specific effect of organic acids is also addressed.