The relation between the corrosion rate of steel in carbonic acid and the CO2 partial pressure was determined by means of weight loss and polarization resistance measurements. This relation is compared with that expected for completely dissociated acids, and appears to be significantly different. A theoretical analysis of this effect shows that it can be quantitatively explained if the direct reduction of undissociated carbonic acid is the rate-determining step in the cathodic process. Both anodic and cathodic processes have been further studied by means of potentiodynamic polarization curves. The effect of temperature on the corrosion rate can be analyzed in terms of an activation energy for the overall corrosion process.