Abstract. 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/N2 mainly results in the formation of nitrites, as identified by PM-IRRAS. O2 hardly reacts with adsorbed nitrites in the absence of NO in the gaseous atmosphere. In contrast, the introduction of O2 to a NO/N2 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 O2 may suggest that nitrites react with gas phase NO2 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 (3.4) is reported for the first time.