The oxidations of sulfate and iodide in the interfacial layers of aqueous microdroplets exposed to O-3( g) for 1 ms are investigated by online mass spectrometry of the electrostatically ejected anions. S(IV) oxidation losses in Na2SO3 microdroplets are proportional to [ S( IV)] [ O-3( g)] up to similar to 90% conversion. I- is more abundant than HSO3- in the interfacial layers of equimolar (Na2SO3 + NaI) microdroplets and similar to 3 times more reactive than HSO3- toward O-3(aq) in bulk solution, but it is converted with minimal loss to I-3(-) and IO3- plus a persistent ISO3- intermediate. These observations reveal unanticipated interfacial gradients, reactivity patterns and transport phenomena that had not been taken into account in previous treatments of fast gas - liquid reactions. (c) 2008 Elsevier B. V. All rights reserved.