Anodic oxidation of thin aluminum and tantalum films was performed in an electrochemical droplet cell. Thicknesses from 2 up to 8 nm were employed on 10 nm thick aluminum and tantalum films. Under ultrahigh vacuum conditions the conductivity of the remaining metal film is monitored as function of time. Equally prepared oxide films were accomplished in metal-insulator-metal capacitors, which were used to monitor the capacitance as a function of time. The combination of the two experimental setups shows clearly, that small losses in the capacitance of aluminum oxide capacitors are due to slight thickening of the oxide even under vacuum conditions. Up to one monolayer of the base aluminum electrode can be oxidized during 10(5) s. Modeling in terms of migration of either oxygen anion interstitials or metal cation migration points preferably to the latter one. Tantalum oxide films of the same thickness show a much larger stability indicating either a lower content of mobile ions or a higher activation barrier for ionization and migration.