Photoactivated reactions of organic species in atmospheric aerosol particles are a potentially significant source of secondary organic aerosol material (SOA). Despite recent progress, the dominant chemical mechanisms and rates of these reactions remain largely unknown. In this work, we characterize the photophysical properties and photochemical reaction mechanisms of imidazole-2-carboxaldehyde (IC) in aqueous solution, alone and in the presence of isoprene. IC has been shown previously in laboratory studies to participate in photoactivated chemistry in aerosols, and it is a known in-particle reaction product of glyoxal. Our experiments confirmed that the triplet excited state of IC is an efficient triplet photosensitizer, leading to photosensitization of isoprene in aqueous solution and promoting its photochemical processing in aqueous solution. Phosphorescence and transient absorption studies showed that the energy level of the triplet excited state of IC (3IC*) was approximately 289 kJ/mol, and the lifetime of 3IC* in water under ambient temperature is 7.9 μs, consistent with IC acting as an efficient triplet photosensitizer. Laser flash photolysis experiments displayed fast quenching of 3IC* by isoprene, with a rate constant of (2.7 ± 0.3) × 109 M-1 s-1, which is close to the diffusion-limited rate in water. Mass spectrometry analysis showed that the products formed include IC-isoprene adducts, and chemical mechanisms are discussed. Additionally, oxygen quenches 3IC* with a rate constant of (3.1 ± 0.1) × 109 M-1 s-1.