We have investigated the reactivity of photolytically generated ground-state, gas-phase atomic oxygen O(3P) with the surfaces of two related ionic liquids, 1-ethyl-3-methylimidazolium ([emim]) bis(trifluoromethyl-sulfonyl)imide ([NTf2]) and 1-dodecyl-3-methylimidazolium ([C12mim]) [NTf2]. The liquids differ only in the length of the alkyl side-chain on the cation. Laser-induced fluorescence was used to detect the gas-phase OH radicals formed at the gas-liquid interface. The considerable yield of OH from [C12mim][NTf2] in comparison to the undetectable amount from [emim][NTf2] cannot simply be explained by the difference in stoichiometry. We propose that this "chemical probe" method therefore provides strong evidence for the preferential accumulation of longer alkyl chains at the gas-liquid interface. © 2009 American Chemical Society.