The effect of dissolved carbon dioxide on steel corrosion in aerated solutions has been variously reported, ranging from "no effect'' to "catalytic enhancement of corrosion. ''A corrosion model was developed, verified with published experimental data, and then used to interpret the role of carbon dioxide in the presence of oxygen. Carbon dioxide has little effect on the corrosion rate when its pressure is less than the oxygen pressure. The model reveals that the effect becomes significant only when the carbon dioxide pressure is substantially higher than the oxygen pressure. The results also show that for a thick boundary layer, carbon dioxide hydration controls the corrosion rate at low oxygen pressures. As oxygen pressure is increased, oxygen dominates corrosion with oxygen diffusion contributing to and then controlling the corrosion rate. For a thin boundary layer, charge transfer of oxygen reduction controls the corrosion rate.