We report the application of the quartz crystal microbalance (QCM) to the in situ study of the oxidation of aluminum in a solution of ethylene glycol and ammonium pentaborate. The mass to charge ratio of the oxide film measured by the QCM during the oxidation varied between 8.2 and 6.6 g equiv-1, suggesting the growth of an A1203 film. The composition of the film was confirmed by XPS. The density of the film was determined from the QCM data and ellipsometric measurements. The obtained value of 3.18 ± 0.02 g cm-3 is in excellent agreement with the literature. These measurements represent the first use of the QCM to study quantitatively the growth and dissolution of thick passive oxide films. © 1989, The Electrochemical Society, Inc. All rights reserved.