A method is provided in which 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0^5,90^3,11]-dodecane (CL-20 or HNIW) is crystallized to its -polymorph by an inverse precipitation technique. A dry CL-20 solvent solution containing an amount of CL-20 dissolved in a CL-20 solvent is prepared. The dry solvent solution is added to a crystallizer containing a CL-20 non-solvent to cause precipitation of epsilon polymorph CL-20 crystals by the inverse precipitation technique. The precipitated epsilon polymorph CL-20 crystals are separated from the non-solvent and the solvent.