Abstract: A process for producing fine-grained .beta.-HMX wherein a solution of .beta.-HMX is treated in a .gamma.-lactone in a temperature range of between 5.degree. and 15.degree. C. with toluene, and the desired fine and very pure .beta.-HMX crystals are precipitated. The resultant crystals have grain sizes of down to 5 .mu.m and exhibit an extremely small proportion of other modifications or forms of HMX. An additional aspect of the process resides in providing the very fine .beta.-HMX with a coating of a thermoplastic polymer. In this procedure, the .beta.-HMX particles, after having been separated from toluene, are made into a slurry in water and combined under agitation at temperatures of between 25.degree. and 60.degree. C. with a solution or suspension or emulsion of the thermoplastic polymer. During this step, granules are formed which can be separated after distillation of the solvent.