The effects of water vapor on the oxidation kinetics of UHV-deposited Al films have been investigated using in situ ellipsometric measurements. For a given exposure time, the presence of ~33% H2O increases the Al2O3 thickness by ~50% and roughly doubles the extinction coefficient of the oxide. We attribute the nonzero extinction coefficients (with a range in values from 0.05 to 0.18) as being due to the presence of excess Al in the oxide. The “wet”-grown Al2O3 layers oxidize at least twice as fast as “dry”-grown layers on storing of samples in a normal laboratory environment. Furthermore, the relative effectiveness of the passivation provided to the underlying Al is greater for the “dry”- than the “wet”-grown AI2O3 layers. Gravimetric and electrical resistance data from samples exposed to an aqueous corrosion test are given to support this. © 1981, The Electrochemical Society, Inc. All rights reserved.