The splitting of DSC melting endotherms of thermostable trioxane-dioxolane copolymers obtained with boron trifluoride: acrylonitrile complex as initiator, which were recorded before and after the annealing of samples at 140°C, was believed to result from an amorphous-crystalline coupling phenomenon brought about by noncrystallizable dioxolane fragments. Quantitative interpretations were made based on the total melting enthalpy and splitting factors after precipitation of dioxolane sequences as crystalline defects by annealing. The results obtained were compared with similar data registered for a commercial trioxane containing copolymer, namely Hostaform C.