The anaerobic thermal decomposition for 2',5'-dideoxyadenosylcobalamin (2'-dAdoCbl) with an added 2,2,6,6-tetramethylpipyridinyl-1-oxy(TEMPO) radical trap in ethylene glycol bas been studied. The temperature dependence of the rate for 2'-dAdoCbl Co-C homolysis was obtained from 85-115°C, AH* = 36.8 ± 0.9 kcal · mol-1, and ΔSor/(+)=22.1±1.5 cal · mol-1 · K- 1. The temperature dependence of the axial-base equilibrium of 2'-dAdoCbl in ethylene glycol was independently deter mined from 10-75°C, ΔH = -6.9±0.2 kcal · mol-1, and ΔS=-18.2±0.6 cal · mol-1 · K-1. The Co-C bond dissociation energy was estimated at 32.4 ± 1.5 kcal · mol-1 for 2'- dAdoCbl in ethylene glycol if one assumes a cage efficiency factor Fc = 1. Comparing the Co-C BDE of 2'-dAdoCbl with that for 5'-deoxyadenosylcobalamin (30.0 ± 2 kcal · mol-1), the cage chemistry effect probably can account for this ~ 2 kcal · mol-1.