The intermediates ISO_3^- (m/z = 207) and IS2O3- (m/z = 239) generated in aqueous (NaI/Na_2S2O_3) microdroplets traversing dilute O_3 gas plumes are detected via online electrospray mass spectrometry within ∼1 ms, and their stabilities gauged by collisionally induced dissociation. The simultaneous detection of anionic reactants and the S_2O_6^(2-), HSO_4^-, IO_3^-, and I_3^- products as a function of experimental conditions provides evidence of genuinely interfacial reaction kinetics. Although O_3(aq) reacts about 3 times faster with I- than with S_2O_3^(2-) in bulk solution, only S_2O_3^(2-) is significantly depleted in the interfacial layers of [I^-]/[S_2O_3^(2-)] = 10 microdroplets below [O_3(g)] ∼ 50 ppm
