Products and intermediates of the oxidation of aqueous alkanoic acids initiated by gas-phase hydroxyl radicals, center dot OH(g), at the air-water interface were detected by mass spectrometry in a novel setup under various experimental conditions. Exposure of submillimolar RCOOH (R = methyl, n-pentyl, n-heptyl) aqueous microjets to similar to 10 ns center dot OH(g) pulses from the 266 nm laser flash photolysis of O-3(g)/O-2(g)/H2O(g) mixtures yielded an array of interfacial species that were unambiguously and simultaneously identified in situ by online electrospray mass spectrometry. We found that peroxyl radicals R(-H)(COO-)OO. react within 50 mu s to produce alcohols R(-H)(COO-)OH and carbonyls R(-2H)(COO-)=O via competitive Russell and Bennett-Summers mechanisms. We confirmed the formation of hydropercoddes R(-H)(COO-)OOH in experiments performed in D2O. To our knowledge, this is the first report on the prompt and simultaneous detection of products and peroxyl/peroxide intermediates in the heterogeneous oxidation of aqueous organics initiated by OH(g).