A method of crystallizing an epsilon-polymorph of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0.sup.5,90.- sup.3,11]-dodecane (CL-20). The method comprises combining the CL-20, at least one organic solvent, and at least one nonsolvent to form a crystallization solution. The crystallization solution is saturated and heated to a temperature greater than about 60.degree. C., such as from about 71.degree. C. to about 94.degree. C. The organic solvent is removed from the crystallization solution while retaining a sufficient amount of the nonsolvent to crystallize the CL-20 as the epsilon-polymorph. The nonsolvent is separated from the epsilon-polymorph of CL-20. A composition of CL-20 comprising the epsilon-polymorph having a particle density of 2.035 g/ml is also disclosed.