<h2>Summary</h2> Highly oxygenated organic molecules are a key component of atmospheric secondary organic aerosol. The origin and formation mechanism of highly oxygenated organic molecules with high unsaturation (HU-HOMs) commonly observed in the atmosphere, however, remain unknown. Through molecular-level investigations of the photochemical aging of soot by O<sub>2</sub> in the air, we find that the multigenerational photo-oxidation of large polycyclic aromatic hydrocarbons (PAHs) on soot by singlet oxygen (<sup>1</sup>O<sub>2</sub>) and superoxide anion radicals (O<sub>2</sub><sup>⋅−</sup>) can be an important source of the unexplained HU-HOMs. The PAH-derived HU-HOMs exhibit lactone and anhydride functional groups and can substantially increase the hydrophilicity of soot. As PAHs are a characteristic and ubiquitous component of combustion aerosols, their photochemical oxidation can generate substantial amounts of HU-HOMs, influencing the atmospheric fate and effects of combustion aerosols.