Atmospheric sulfate extracted from rainwater and aerosols has been sampled from two environments: La Jolla and White Mountain Research Station (WMRS), California. In general, the multiple stable oxygen isotopic analysis of these sulfate samples shows that they possess mass-independent oxygen isotopic compositions, unlike laboratory nonatmospheric standards. As suggested by the recent laboratory findings, the source of mass independent oxygen isotopic composition in sulfate is implied to be aqueous phase S(IV) oxidation by atmospheric H2O2 and O3. The relative S(IV) oxidation contribution by H2O2 and O3, which together generally represent the overall aqueous phase S(IV) oxidation, is estimated utilizing the average 17O enrichment value of La Jolla rainwater sulfate samples as an index. The estimate indicates a relative dominance of the aqueous over gas phase S(IV) oxidation. Although inconclusive owing to small data sets, both La Jolla and WMRS aerosol sulfate samples display seasonal 17O enrichment variations with a maximum in winter and a minimum in summer. Such seasonal isotopic variations are believed to derive from climatic effects that favor aqueous phase S(IV) oxidation in winter compared to summer. The radioactivity measurements of the cosmogenic 35S radionuclide, coupled with double oxygen isotope ratio measurements suggests an atmospheric sulfate contribution from high-altitude air mass (perhaps stratospheric) at WMRS, in particular during late winter/early spring season. The results of this work provide a new way to quantify relative oxidative pathways of S(IV) species in the atmosphere and to improve understanding atmospheric processes associated with sulfate. ¿ 2001 American Geophysical Union