The thermolysis and 350-nm and 185-nm photolyses of the azoalkanes 2,3-diazabicyclo[2.2.l]hept-2-ene (la) and spiro[cyclopropane-1,7'-[2,3]diazabicyclo[2.2.l]hept-2-ene] (lb) have been investigated. The exo/endo stereochemistry in the bicyclo[2.l.Q]pentanes 2a,b and in the rearranged olefin 3b w a~ determined by deuteration experiments using 5,6-exo-dideuterioazoalkanes la,b-d2. Whereas thermal and direct photochemical (350 nm; n, T * ) denitrogenation of azoalkane la-d2 led exclusively (>99%) to bicyclo[2.1.0]pentane 2a-dz with preferential (1.54, 2.94) double inversion, the triplet-sensitized photolysis afforded nearly complete stereoequilibration. In 185-nm denitrogenation an unexpectedly high exo/endo ratio (3.1) for bicyclo[2.1.O]pentane 2a-d, was found, besides isomerization to cyclopentene 2a-dz. Similar results were obtained in the denitrogenation of spiroazoalkane lb-d2, which exhibited exo stereochemical preferences in both photoproducts spiro[bicyclo[2.1.0]pentane-5,1'-cyclopropane] 2b-d2 and bicyclo[3.2.0]hept-l-ene 3b-d2. The 350-nm photolysis of azoalkane lb-d, gave preferential formation of ezo-spirobicyclo[2.1.O]pentane 2b-d2 and exo-olefin 3b-d2, whereas triplet-sensitized decomposition yielded almost complete loss of stereochemical preference in the olefin 3b-dz. The 185-nm photolysis of azoalkane lb-d2 showed similar behavior compared with the azoalkane la, e.g., a t high exo/endo ratio in spirobicyclo-[Z.l.O]pentane 2b-d2. Also olefin 3b was formed with complete stereoequilibration. These diverse experimental results are discussed in terms of one-bond vs. two-bond cleavage processes leading to the diazenyl diradicals D'n,,,and D'u,r in the case of the low-energy activation (350-nm photolysis and thermolysis) and 1,3-cyclopentadiyls D , , and D , , on high-energy activation (185-nm activation). The relatively high degree of double inversion in the corresponding bicyclopentanes and the formation of rearranged cycloalkenes in the 185nm photodenitrogenation is presumably a direct consequence of concerted two-bond cleavage via the formation of IDu,. and zwitterionic states of the 1,3-diradical. A Salem diagram for one-bond and two-bond denitrogenation was most helpful in rationalizing these results mechanistically.