In situ diffusion reflectance infrared Fourier transform spectroscopy was used to study the water adsorption on mineral oxides (SiO2, alpha-Al2O3, MgO, Fe2O3, TiO2). The results showed that all the water adsorption isotherms were well fitted with the Brunauer-Emmett-Teller (BET)-III type equation, with the calculated monolayers occurring at 24%-30% relative humidity. It showed that about 1-5 layers of water adsorbed on oxides surfaces in ambient relative humidity (20%-90%). The measured deliquescence relative humidity of NaCl was (74 +/- 1)%, which demonstrated that DRIFTS is a useful method for the study the hygroscopic behavior of mineral dust. In addition, the limits of DRIFTS were also discussed.