Computed singlet ground-state and low-lying singlet and triplet excited-state potential energy surfaces of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) and 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) are combined with experimental facts and reasonable models for how excess energy is partitioned among internal and solvent modes to propose mechanisms for the title reactions. In particular, we put forth a mechanism that is consistent with the fact that DBO produces bicyclo[2.2.0]hexane (BCH) and 1,5-hexadiene (HD) in very low quantum yield and in proportions dependent on the spin state of the excited species while DBH produces bicycle[2.1.0]pentane with near unit efficiency from either singlet or triplet excitation. Our mechanism also rationalizes heavy atom effects on the product ratios in DBO and the stereospecificity and HD:BCH yield ratios seen in the products.