In this study, transmission FT-IR and diffuse reflectance UV-vis spectroscopies were used to probe the heterogeneous chemistry of gaseous nitrogen dioxide on mineral oxide particles. In particular, the role of pre- and post-adsorbed water on the heterogeneous chemistry of NO2 on Al2O3 and TiO2 was investigated. At 298 K, NO2 reacts with the surface of these mineral oxides to form surface nitrite and nitrate. The spectroscopic data show that two distinct types of surface nitrate exist, oxide-coordinated and water-solvated nitrate. The relative amounts of oxide-coordinated and water-solvated nitrate are dependent on the coverage of adsorbed water on the surface of the mineral oxide particle. The importance of this water layer in the heterogeneous atmospheric chemistry of mineral oxide aerosol is discussed. ¿ 1998 American Geophysical Union