The sulfuric acid-induced corrosion of smooth (2 nn average roughness) aluminum surfaces has been studied in real times using an in situ Fourier transform infrared reflection absorption spectrometer and a quartz crystal microbalance. Submicron thick, 35 to 55 weight percent (5 to 12 molal), sulfuric acid films were formed on room temperature metal surfaces by the reaction of gas-phase SO3 and H2O vapor in a flowing gas system at a total pressure of similar to 200 Torr. The deposition of the acid films and subsequent changes in their chemical composition resulting from corrosion of the aluminum substrate could be monitored using characteristic infrared absorption features. The corrosion process always significantly perturbed the spectral signature of the films from that which was observed on inert gold surfaces. Using changes in spectral features that are linked to the production of Al3+ as indicators of corrosion, we conclude the rate of corrosion of the metal is strongly enhanced by both higher relative humidities and increased rates of sulfuric acid deposition.