A new polycyclic nitramine, hexanitroisowunzitane (HNIW), has physical properties which differ from monocyclic nitramines. One unique property of HNIW is the formation of inclusion sites in the a-polymorph which can trap solvents or other impurities, forming clathrate complexes. Trapped solvents such as methanol and H2O have been observed directly by crystal structures of α-HNIW. Free radicals such as (i) nitric oxide and nitrogen dioxide, (ii) nitroxides, (iii) radical pairs, and (iv) ionic or non-ionic paramagnetic fragments can be formed in crystalline nitramines by photolytic and thermal decomposition. In a previous EPR (electron paramagnetic resonance) study, trapped NO2radicals have been analyzed in a single crystal of HNIW at 77 K before (I) and after (II) annealing an HNIW crystal at 298 K. In this study, the formation of a free radical product fromthermal decompositionof HNIW is reported. The radical is detected in the temperature range from 403–443 K by heating HNIW in sulfolane (b.p. 558 K) or deuterated dimethylsulfoxide (b.p. 458 K). The radical product is suggested to form from the parent fragment after NO2dissociates.