A variety of artificial fibers extensively employed as lining in high-temperature apparatus may undergo a devitrification process that leads to significant changes in the chemical-physical properties of the materials. Among them, the crystallization of carcinogenic minerals, such as cristobalite, has already been documented for alumino-silicate ceramic fibers. Five fibrous samples with different compositions were treated over a wide range of temperatures (20-1500°C) and times (24-336 h) to investigate the rate and the crystalline phases that are formed as well their onset temperatures. The new phases were characterized by using a multimethodic approach: phase transformations were monitored together with thermal analysis and the new phases were investigated by using X-ray powder diffraction analysis. The crystalline:amorphous ratio was monitored by Rietveld refinement of X-ray diffraction data. Scanning electron microscopy was used to study the effect of heat treatments on the morphology of fibers, and the nanostructures were investigated by transmission electron microscopy (TEM). The results show that the main crystalline phases are cristobalite, diopside, mullite, and zirconia. The onset of cristobalite was observed at temperature lower than that thermodynamically expected. The TEM analysis showed that protostructures were present in the material vitrified from sol-gel-derived products, which can act as crystallization nuclei. The study shows that the devitrification leads to health hazard due to the formation of inhalable powder of cancerogenic crystalline phases.