Ice cores have provided a wealth of information about past atmospheric composition and climate variability. However, relatively little is known about how the chemistry of the atmosphere has responded to natural climate change and anthropogenic influences. The oxygen isotopes (δ17O and δ18O) of sulfate serve as a recorder of the relative amounts of gas and aqueous-phase oxidation pathways in the atmosphere. This quality, along with its stability, renders sulfate an ideal proxy to investigate changes in oxidation pathways of S(IV) species in present and ancient atmospheres. The oxygen isotopic composition of sulfate in eight samples from the Vostok, Antarctica ice core, covering one full climate cycle, is presented. Assuming tropospheric-derived sulfate only, isotope data reveal that the ratio of gas-phase over aqueous-phase oxidation of S(IV) species was greater during the last glacial than the surrounding interglacial periods.