Field experience has shown that CO2 corrosion is considerably reduced at low partial pressures, unless more than 0.1 to 1 mM of acetic acid is present in the water. This paper shows that acetic species actually act as a weak inhibitor of the anodic dissolution reaction, and that the role of acetic acid is clearly related to an associated inversion of the acetate/bicarbonate ratio, together with the resulting difficulty in precipitating protective iron carbonate. In addition, despite its minute concentration, acetic acid becomes the main source of the acidity consumed by corrosion. In such conditions, a genuine acetic acid corrosion occurs, controlled by a volubility equilibrium with a gas phase containing acetic acid vapor, like in the case of CO2 corrosion.