An attempt was made to introduce CL-20 in a TNT/energetic thermoplastic elastomer (ETPE)-based melt cast formulation, to obtain an insensitive composition with reduced adverse environmental properties. A loading limit of 42% w/w of CL-20 in melted TNT was observed, while it should have been around 70%. This paper describes the investigation that was undertaken to understand the observed phenomena. It was demonstrated that CL-20 undergoes structural alterations in melted TNT. The relative solubility of CL-20, RDX and HMX in melted TNT was determined and the alpha-, beta-, gamma- and epsilon-CL-20 were prepared and characterized using Raman spectroscopy and DTA. CL-20 was mixed in melted TNT, as much as 4.2 g of CL-20 dissolved in 100 g of TNT. This allowed a transformation from epsilon to beta-CL-20 and a modification of CL-20 particle size and distribution. The later modification induced a raise in CL-20 specific surface and was responsible for the loading limit, while the transformation to beta- CL-20 caused an increase in sensitivity and a decrease in density. This indicates that the use of CL-20 in TNT melt cast is not promising. Our study also showed that CL-20 is sensitive to morphological transformations, which should be taken into consideration in future processing using this compound.