In this study, we investigated aqueous-phase triplet excited states (3C⁎)-induced photo-degradation of 4-ethylguaiacol (EG) under both simulated sunlight and ultraviolet (UV) light irradiations. Through quencher experiments, the relative contributions of reactive oxygen species (ROS, such as 1O2/O2-/·OH) and 3C⁎ were calculated and results showed three reactive species, e.g., 3C⁎, 1O2 and O2-, all seemed to play important roles in the photo-degradation of EG, but contribution from ·OH was relatively minor. High steady-state 1O2 concentration after 1 h irradiation further revealed the major contribution of 1O2 to photo-degradation under Xe light irradiation. The degradation experiment under three saturated gases (air, O2 and N2) showed that the degradation rate in air-saturated condition was the largest owing to synergistic effect of 1O2 and 3C⁎. Oxidative capacity of aqueous secondary organic aerosol (aqSOA) increased with reaction time by monitoring oxygen-to‑carbon (O/C) ratio and carbon oxidation state (OSc) via an aerodyne soot particle aerosol mass spectrometer (SP-AMS). Moreover, aqSOA mass yields were calculated via SP-AMS data. The UV-vis spectral change suggested formation of light-absorbing organics at first stage under simulated sunlight irradiation. Based on the identified products and the reactive intermediates, we postulated that 3C⁎-induced oxidation might be attributed to direct reactions by 3C⁎ and 1O2, chemical reaction by ROS, as well as oligomerization via H-abstraction. To the best of our knowledge, this is the first time to explore systematically reaction pathways of 4-ethylguaiacol under 3C∗ radical on the basis of thorough analysis of products and reactive species. Our findings highlight the impacts of aqSOA from biomass burning emissions on air quality and climate change.