Process for producing the epsilon polymorphic form of hexanitrohexaazaisowurtzitane.According to the process which is the subject-matter of the invention, a saturated solution of hexanitrohexaazaisowurtzitane of any polymorphic form is prepared in a mixture comprising a solvent chosen from esters, nitrites, ethers and ketones, except acetone, and a non-solvent chosen from aliphatic hydrocarbons and aromatic hydrocarbons, the solvent being more volatile than the non-solvent.This saturated solution is subsequently seeded with a few crystals of hexanitrohexaazaisowurtzitane of epsilon polymorphic form and then the mixture is concentrated by evaporation of the solvent.This process is simple, is economical and can be easily extrapolated to the industrial stage.It makes it possible to obtain particle size fractions within the 10 .mu.m-100 .mu.m range with smooth grains with regular facies.Hexanitrohexaazaisowurtzitane, in particular the densest epsilon form, is useful as oxidizing or explosive charge in pyrotechnic compositions.