Mineral dust represents one of the largest sources for the global tropospheric aerosol. It has been found that aged mineral dust particles are enriched in nitrate compared to young aerosols which may be due to heterogeneous reactions of HNO3 or N2O5 with the particles (Carmichael et al., 1996). Here we present a study of the heterogeneous reactions of H2O and HNO3 on Saharan dust originated from deposition on the Cape Verde Islands during the Holocene (Rognon et al. 1996). Using Diffuse Reflectance IR Fourier Transform spectroscopy (DRIFTS) the formation of surface species, such as particulate nitrate, was monitored and insight in the reaction mechanism is obtained. Addnl., H2O and HNO3 uptake coeffs. were detd. with a Knudsen cell reactor coupled to a quadrupole mass spectrometer. The combination of these two expts. provides a mean to analyze the gas phase as well as the condensed phase during the heterogeneous reaction and compare the results.