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The formation of brown carbon (BrC) in aqueous atmospheric aerosols is well-documented and often attributed to aldehyde-ammonia reactions. However, many studies have focused on individual aldehyde precursors, overlooking the complex composition of organic aerosols, which comprise a diverse mix of organic and inorganic compounds. To address this, a complex BrC system was investigated by generating aqueous atmospheric aerosol mimics containing glyoxal (Gly), glycolaldehyde (GAld), and ammonium sulfate. Structural analysis using supercritical fluid chromatography−mass spectrometry (SFC-MS) showed that adjusting the Gly:GAld mole ratio leads to variations in the composition and abundance of BrC products formed. Notably, aromatic heterocycles (e.g., imidazoles and pyrazines) as well as acyclic carbonyl oligomers were identified to form at different concentrations depending on the Gly:GAld mole ratio. UV−visible spectroscopy analysis demonstrated that light absorption in these mixed Gly + GAld + AS systems cannot be modeled as a simple weighted average of the Gly:GAld mole ratio; observed changes in light absorbance can be explained by compositional changes in solution. These observations indicate that cross-reactions are occurring between the Gly and GAld in solution, potentially leading to changes in the physical properties of the aerosol. Given the thousands of reactive compounds found in atmospheric aerosol, these findings could have important implications for our understanding of organic reactions within the aerosol.