The formation and configuration of sulfate species on Ag/Al2O3 were studied by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. The comparison between theoretical and experimental vibration spectra enable us to draw the following conclusions: Bidentate sulfate species rather than tridentate sulfate species are the predominant surface species on Ag/Al2O3. Moreover, both bidentate and tridentate sulfate species may coexist on the catalyst surface at a lower coverage. The accumulation of surface sulfate species could well explain the blue shift of the sulfate species in IR spectra. In addition, the in situ DRIFTS could distinguish between the sulfate species that linked to Al site and Ag site, which was well supported by temperature-programmed desorption (TPD) results.