A laser technique was used to initiate single corrosion pits as well as geometrically ordered arrays of pits on Al (99.999%) immersed in 1M NaCl at pH 11. The current was measured during potentiostatic growth and, upon termination of each experiment, the shape of each pit was recorded. It was found that single pits had round, polished surfaces for applied potentials between -0.7 and -0.52 V SCE, and for growth times up to 90 s. During the first 30 s of growth, the current from polished hemispherical single pits was found to be independent of the applied potential. Experimental measurements of pit radius and current density as a function of time agreed with calculations based on diffusion controlled dissolution of an aluminum oxychloride surface film. Current interruption experiments demonstrated that pit stability depended on the pit size, the duration of time lapse at open circuit, and upon the applied potential. These results were interpreted to mean that, at a particular applied potential, a critical concentration adjacent to the pit surface is required if the pit is to remain active. From potential step measurements, it was found that there was a critical applied potential below which pits repassivated, and that the critical potential decreased with pit size; these results suggest the hypothesis that a necessary condition for repassivation was the dissolution of a surface film. For arrays of pits grown under galvanostatic conditions, it was observed that the smallest pits repassivated first.