The complex formation between HOO and SO 2 in argon matrices has been studied with FTIR spectroscopy. The complex shifts of the intramolecular fundamentals of the complex components suggest the formation of a complex, where HOO acts as a lone pair donor to the sulfur atom of SO 2 while its hydrogen atom is sufficiently close to one of the oxygen atoms of SO 2 to give a significant blue shift of the HOO bend. The complex is rapidly decomposed by irradiation at 266 nm. A comparison between the spectrum of the photoproduct and the spectrum of matrix isolated sulfuric acid suggests the possibility that the photoproduct is the radical formed by dissociation of one of the OH bonds of sulfuric acid.