The stability with respect to spin‐forbidden radiationless decay of the previously reported [J. Chem. Phys. 88, 7248 (1988)] asymmetric dimer of NO, N–N–O–O (a‐N₂O₂) is considered. The spin‐allowed decay channel a‐N₂O₂(¹ A’)→N₂O(X  ¹Σ⁺)+O(¹ D) is endoergic. However, the spin‐forbidden decay channel a‐N₂O₂(¹ A’)→N₂O(X  ¹Σ⁺)+O(³ P) is exoergic. Large scale multireference configuration interactionwave functions, approximately 300 000–1 400 000 configuration state functions, based on double zeta polarization and triplet zeta polarization bases are used to study this process. The minimum energy crossing of the ground singlet ¹ A’ state and the lowest excited triplet ³ A‘ state was determined as was the interstate spin–orbit coupling. This electronic structure data was used in the context of a simple one‐dimensional model to show that a‐N₂O₂ is rapidly predissociated to N₂O(X  ¹Σ⁺) and O(³ P).