The intermediates ISO3- IS2O3- (m/z = 207) and (m/z = 239) generated in aqueous (Nal/Na-2)S2O3) microdroplets traversing dilute 03 gas plumes are detected via online electrospray mass spectrometry within similar to 1 ms, and their stabilities gauged by collisionally induced dissociation. The simultaneous detection of anionic reactants and the S2O62-, HSO4-, 103, and 13 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 S2O32- in bulk solution, only S2O32- is significantly depleted in the interfacial layers of [I-]/[S2O32-] = 10 microdroplets below 103(g)] similar to 50 ppm.