Significance Despite its enormous evolutionary success (it is the carboxylating enzyme of all photosynthetic pathways from microorganisms to higher plants), Rubisco is rather inefficient due to wasteful competitive inhibition by molecular oxygen. Quite critically, the intimate mechanism of O ₂ addition is unknown. We show here that isotope effects ( ¹³ C, ¹⁸ O, ²⁵ Mg, or ² H) and high level computations of redox potential are all consistent with oxygen acting as an oxidant in a redox reaction generating superoxide which then attacks the substrate. Our results explain why the elimination of oxygenation by enzymatic bioengineering is so difficult, because it would require a drastic change in electrostatic and/or redox potential of the substrate, and this would alter carboxylation activity.