Abstract. Small -dicarbonyls represent the major precursors of secondary organic aerosol (SOA) and brown carbon (BrC) in the atmosphere, but the chemical mechanisms leading to their formation remain unclear. Here we elucidate the fundamental kinetics and mechanisms for aqueous-phase oligomerization of glyoxal (GL) using quantum chemical and kinetic rate calculations. Our results identify several essential isomeric processes for GL, including protonation to yield diol/tetrol and carbenium ions, nucleophilic addition of carbenium ions to diol/tetrol as well as to free methylamine/ammonia (MA/AM), and deprotonation to propagate oligomers and N-heterocycles. Both protonation and nucleophilic addition occur without activation barriers and are dominantly driven by electrostatic attraction. Deprotonation proceeds readily via water molecules in the absence of MA/AM but corresponds to the rate-limiting step for N-containing cationic intermediates to yield N-heterocycles. On the other hand, the latter occurs readily via a catalytic process by acidic anions (e.g., SO42-). A carbenium ion-mediated reaction rate of GL is 4.62103s1 under atmospheric conditions, in good agreement with the experimental data. Our results provide essential mechanistic and kinetic data for accurate assessment of the role of small -dicarbonyls in SOA and BrC formation.