SO2 is an atmospheric pollutant whose oxidation leads to acid rain, while alpha -Fe2O3 (hematite, a naturally occurring component of atmospheric aerosol particles) has a charge-transfer band gap (about 2.2 eV) smaller than the cutoff of solar radiation in the troposphere (about 4.3 eV) and is thus able to participate in photochemical reactions. The interaction of SO2 with UV-irradiated, single-crystal alpha -Fe2O3 was examined by using UPS, XPS, and Auger spectroscopies. Between 261 and 331 K, SO2 adsorbs on the alpha -Fe2O3(0001) surface with a very low sticking coefficient in the absence of UV irradiation. The adsorbed species resembles SO42-; its heat of adsorption is estimated to be 2.4 eV. UV irradiation of the alpha -Fe2O3 substrate during SO2 exposure leads to a significantly increased rate of adsorption. All of the cations in stoichiometric alpha -Fe2O3 are Fe3+; UV irradiation produces Fe2+ cations through the creation of electron-hole pairs. The Fe2+ sites are found to be much more reactive to SO2 adsorption than are Fe3+ sites. A model is proposed in which SO2 adsorbs only at Fe2+ sites. According to this model, the small number of adsorption sites which are present in thermal equilibrium are greatly enhanced by W irradiation, leading to an increased rate of SO2 adsorption.