In the atmospheric corrosion of copper, nickel, and iron, the adsorption of water affects the corrosion rates. Knowledge of water adsorption and metal oxyhydroxide formation is important in understanding the atmospheric corrosion process. The adsorption of water on copper, nickel, and iron in humid air is accompanied by oxide formation, and the two processes were separated by combining mass measurements with measurements of the quantity of oxide formed. The thickness of the oxide formed in humid air on copper, nickel, and iron was measured ex situ by coulometric reduction technique. The thickness of the oxide film grown in humid air is a function of both relative humidity and temperature. Since oxides on copper, nickel, and iron are semiconductors and they grow by the diffusion of cation vacancies, the increased growth of the oxides in the presence of adsorbed water is explained in terms of the diffusion by cation vacancies.