The oxygen isotopic composition (16O, 17O, and 18O) of sulfate formed from different oxidative reactions has been investigated. In the aqueous phase, sulfur oxidation by H2O2, O3, and O2, catalyzed by Fe(III) and Mn(II) were studied. In the gas phase we have investigated the only relevant reaction for the atmosphere: SO2+OH and its chain termination reaction SO3+H2O. The results show that none of these reactions, gas or aqueous phase, produce a mass-independent oxygen isotopic composition in sulfate. Since H2O2 and O3 are known to possess a mass-independent isotopic signature, we have investigated the possible transfer of this anomaly to sulfate. It appears that both these oxidant species transfer their anomaly. Isotopic analysis shows that two oxygen atoms from H2O2 are found in the product H2SO4. This result is in accord with previous work. For O3 we found that only one of the original ozone oxygen transfers to the product sulfate. These isotopic results contradict the free radical reaction mechanism proposed by Penkett et al. <1979> but agree with the nonfree radical mechanism suggested by Erickson et al. <1977>. Therefore, it appears that only aqueous phase oxidation produces a mass-independent oxygen isotopic composition in sulfate. This finding is a response of the origin of the mass-independent oxygen isotopic composition of atmospheric and mineral deposits of sulfate on Earth <Bao et al., 2000; Lee, 1997>. Furthermore, this finding allows us to quantify the relative proportion of sulfate production by OH (gas phase formation) and by H2O2 and O3 (aqueous phase formation). The results can be used to test atmospheric chemical/transport models. ¿ 2000 American Geophysical Union