The early stages of oxide film passivation of corroding surfaces in aluminum etch tunnels and pits were investigated. Passivation was initiated by step reductions in applied current. A mathematical model f or passivation was used to predict experimentally measured potential transients during the first millisecond after the current step. The experimental transients had a characteristic potential undershoot at about 70 mus after the current step; according to the model, the undershoot was directly related to the partial coverage of the corroding surface with an oxide layer. The time and extent of the undershoot were in quantitative agreement with the theoretical predictions, suggesting that the fractional actively dissolving area in the pit at these times is a linear function of potential and adjusts rapidly to changes of the potential. This reconfiguration of the active area occurs at times when the extent of passive film formation is still small. A chemical mechanism for passivation which is consistent with the model is one in which the fractional active area is controlled by the coverage of specifically adsorbed chloride ions. Agreement between experimental and predicted potential transients was also observed at room temperature, where no tunnels are found, but only corrosion pits with irregular shapes.