The interaction of NO, and the further reactivity of oxygen, with water-passivated aluminum surfaces are investigated for the first time by in situ polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS) coupled with quartz crystal microbalance (QCM). Chemical and quantitative complementary information are thus obtained.Exposure of water-passivated plane aluminum samples to NO/N 2 mainly results in the formation of nitrites, as identified by PM-IRRAS. O 2 hardly reacts with adsorbed nitrites in the absence of NO in the gaseous atmosphere. In contrast, the introduction of O 2 to a NO/N 2 mixture leads to the rapid transformation of nitrites to nitrates, both water-solvated and oxide-coordinated species. The fact that the transformation of nitrites to nitrates occurs only in the presence of NO and O 2 may suggest that nitrites react with gas phase NO 2 to form nitrates and NO. The evolution of the PM-IRRAS spectra of the Al surface with the composition of the gaseous phase is fully correlated to the mass uptake changes measured by QCM.From the PM-IRRAS and QCM measurements obtained during the same experiments, the ratio of the IR response coefficient of nitrates to that of nitrites (3) is reported for the first time.