Imidazoles get growing interest in atmospheric chemistry, since catalytic process in situ involving ammonium salts and glyoxal, two ubiquitous component of ambient particulate matter, could lead to their significant formation [1,2]. Recently, our group showed that imidazole-2-carboxaldehyde (IC), one of the products formed by the ‘iminium-pathway’, can efficiently photo-induce the growth of aerosols by the production of highly oxygenated limonene oxidation products without the presence of any gas phase oxidant [3,4]. Imidazole compounds are also known to be present in the sea surface microlayer (SML), the thin interfacial zone between atmosphere and subsurface water [5] and IC could certainly be one of them. Photoactive compounds in the SML can produce active halogen species after oxidation of halides. Photo-activated aromatic carbonyls, like benzophenone, and other photoactive compounds like chlorophyll, can lead to the formation of the reactive radical anion (X2•-) and molecular halogens, that can be released in the gas-phase [8]. Gas-phase halogen atoms, especially iodine and bromine, will set in catalytic cycles of ozone destruction in the MBL, changing locally the oxidizing capacities and gas-phase iodine chemistry can also lead to particle formation [9]. The aim of this work is to explore the photochemical properties of IC by the means of laser flash photolysis. The absorption spectrum of the triplet state of IC in aqueous solution was measured and the bimolecular rate coefficients of IC in presence of halide anions determined. Iodide anions showed to be efficient quenchers of the triplet state of imidazole-2-carboxaldehyde, with a rate coefficient kq of (5.33 ± 0.25) x 109 M-1 s-1.The quenching by bromide and chloride anions was less efficient, with respective rate coefficients of (6.27 ± 0.53) x 106 M-1 s-1 and (1.31 ± 0.16) x 105 M-1 s-1. The halide (X-) quenches the triplet state by an electron transfer mechanism and the resulting transient absorption feature matches that of the corresponding radical anion (X2-). We suggest that this type of direct photochemical reactions is a driving force of oxidation reactions in the oceanic SML and consequently a significant source of halogen atoms in the atmosphere.