The pyrolysis process of poly[2,4,6-tri(methylamino)borazine]-based fibers has been studied in detail. The condensation of polymer leads to infusible fibers with a three-dimensional polymeric network from 70 to 400 °C. A polymer–mineral transition occurs since 400 to give amorphous hydrogenated fibers at 1000 °C. The chemical conversion process is complete at 1400 °C and the BN fibers crystallize up to 1800 °C. The pyrolysis process of poly[2,4,6-tri(methylamino)borazine]-based fibers has been studied in detail. The condensation of polymer leads to infusible fibers with a three-dimensional polymeric network from 70 to 400 °C. A polymer–mineral transition occurs since 400 to give amorphous hydrogenated fibers at 1000 °C. The chemical conversion process is complete at 1400 °C and the BN fibers crystallize up to 1800 °C.