It is reported that atmospheric (aerosol and rainwater) sulfate possesses a mass independent isotopic composition, with excess 17O. Laboratory experiments indicate that the oxidants, H2O2 and O3, are the source of anomalous 17O enrichments in atmospheric sulfate via aqueous phase S(IV) oxidation. The discovery of the oxygen anomaly in atmospheric sulfate, with definition of its origin, provides a new way to investigate atmospheric and general planetary processes. The observations provide new insight into atmospheric chemistry with application in studies of the evolution of the atmosphere, and surface-atmosphere interactions, both on Earth and Mars. ¿ 2001 American Geophysical Union