The oxygen isotopic composition (Δ17O) of non-seasalt sulfate (NSS) aerosol was measured in samples from Alert, Canada over one year (July 1999--June 2000) and used to quantify the S(IV) oxidants. Measurements of Δ17O in NSS are used to evaluate the relative contributions of O3, H2O2, and OH oxidation leading to the formation of SO42- compared to a model of Feichter et al. (1996). The isotopic values suggest that there is a twofold overestimate of ozone oxidation in the model during winter. The isotopic composition is consistent with 10 to 18% contribution from a non-photochemical oxidation pathway, likely Fe3+/Mn2+-catalyzed O2 oxidation, during the dark Arctic winter. Isotopic evidence also invokes a 3 to 10% contribution of a mass dependent oxidant during springtime Arctic ozone depletion events.