Halogens play key roles in the chemical composition of marine boundary layers, the free troposphere and the stratosphere. Atmospheric halogen chemistry is dominated by reactions between gas-phase and aqueous species on the surfaces of the ocean and marine aerosol. The mechanisms of interfacial halogen radical/halide reactions, however, are not fully understood, partly due to the dearth of techniques for in situ monitoring of the products and intermediates of fast interfacial halogen radical reactions. Here, we report the online electrospray mass spectrometric identification of the species produced on the surface of aqueous Br- and I- microjets collided by I-center dot(g) pulses generated from the 266 nm laser photolysis of CH3I/O-2/N-2 gas mixtures. Mass-specific identification of intermediates and products in D2O and (H2O)-O-18 solutions and their dependences on I-center dot(g) fluxes let us outline mechanisms of formation. We found that the uptake of I-center dot(g) on the surface of Br- and I- microjets (effective uptake coefficient gamma(eff) >= 2 x 10(-4)) yields IBr center dot-/I-2(center dot-) radical intermediates, which rapidly react with additional I-center dot to produce trihalides I2Br-/IBr2-/I-3(-) plus I3On- (n = 1, 2) species within similar to 10 its. Our findings point to a new halogen activation pathway initiated by photogenerated I-center dot.