Abstract. In this study we report the identification of bicyclic imidazoles in aqueous aerosol mimics using HPLC-ESI-MS/MS. 2,2'-Biimidazole was identified to be a major contributor to the 280 nm absorbance band observed in mixtures of glyoxal and ammonium sulfate, despite the fact that its production rate is two orders of magnitude lower than the previously reported production rates of imidazole or imidazole-2-carboxaldehyde. The molar absorptivity of 2,2'-biimidazole was determined to be (36 690 ± 998) M⁻¹ cm⁻¹. This demonstrates the necessity of molecular product identification at trace levels to enable a better understanding of relevant absorbing species. Additionally, the formation of lower polarity products including formamides of imidazoles is proposed. The role of imidazoles and other light-absorbing species in the formation of SOA and optical properties of SOA is discussed and potentially interesting fields for future investigations are outlined.