Cathodic polarization curves for the reduction of oxygen and protons on platinum and iron electrodes under thin electrolyte layers (X-i : 1 mm to 10 mu m) were measured to elucidate the effects of the thickness and the pH of the electrolyte layer on the initial atmospheric corrosion stage of iron. A Limiting current was exhibited on the cathodic polarization curves under neutral electrolyte layers containing 0.1 to 2 M NaCl, tvhich was inversely proportional to the thickness of the electrolyte layer down to approximately 20 mu m. This result indicates that oxygen diffusion through the electrolyte layer is the rate-determining step (rds) for the oxygen reduction process. Meanwhile the limiting current under still thinner layers (X-i : 10 to 20 mu m) was independent of the thickness, where an oxygen dissolution step at the air/electrolyte interface is the rds because of the considerably higher diffusion rate under such very thin electrolyte layers. AC impedance corrosion monitoring of iron for the period of 4 h was performed under electrolyte layers of different thickness (X-i : 1 mm to 10 mu m) and pH (pH 3.0, 4.0, and 5.7). The results indicated that the corrosion rate showed a maximum at an electrolyte thickness of 20 to 30 mu m in all cases, and the effect of pH on the atmospheric corrosion rate was negligible, except for cases wherein a relatively thick electrolyte layer (>1 mm) was present far a short exposure period (<1 h). This can be attributed to a neutralization of the electrolyte due to corrosion reaction because of the extremely Limited amount of electrolyte present.