Ionized azo-t-butane (m/z 142) undergoes three dissociation reactions: competitive cleavage of the N-C bond to form (1) the t-butyl cation ((CH3)(3)C+, m/z 57) plus a neutral that is nominally (CH3)(3)CN2 center dot (85 Da); (2) m/z 85 cation, (CH3)(3)CN2+, plus the neutral t-butyl radical; and (3) a minor rearrangement reaction leading to ionized butene. The competition between channels (1) and (2) is governed by both energetic and entropic considerations as the 85 Da neutral lies in a 1 kJ mol(-1) potential energy well and when formed dissociates into the t-butyl radical and N-2. This gives channel I an entropic advantage over channel 2, as long as the transition states for these processes reside close to products, a conclusion supported by threshold photoelectron photoion coincidence spectroscopy, tandem mass spectrometry, and ab initio calculations.