This work describes the high temperature reaction sequence of commercial Man Made Vitreous Fibers (MMVF) Cerafiber, Superwool, Rock wool and Glass wool which may be used as substitute for asbestos in some industrial applications. Knowledge of the reaction path and transformation sequence is very important to assess whether carcinogenic crystalline phases are formed during devitrification, which may occur when used as insulators. In addition, knowledge about the nature of the phases formed at high temperature is mandatory to assess if thermally transformed MMVF can be safely recycled as secondary raw material (SRM). In this scenario, this study provides useful information for the optimization of the industrial annealing process aimed to attain a safe, recyclable product. The results of this work show that one of the high-temperature products of Cerafiber and Superwool is cristobalite which is classified as a carcinogenic. It was possible to define the temperature interval at which Cerafiber and Superwool fibers can be safely used as thermal insulators (e.g. insulators in tunnel and/or roller kilns, etc.). As cristobalite is formed in both synthetic fiber products at temperatures higher than 1200 degrees C, their use should be limited to devices operating at lower temperatures. Rock and Glass wool melt upon thermal treatment. As far as the industrial process of inertization is concerned, a maximum firing temperature of 1100 and 600 degrees C is required to melt Rock wool and Glass wool, respectively, with the high-temperature products that can be safely recycled as SRM. Recycling of these products in stoneware tile mixtures were subsequently attempted. The addition of 1-2 wt.% of the melts of Rock and Glass wool gave promising results in terms of viscous sintering reactions and resistance to staining with the only weak characteristic being the color properties of the fired bodies which tend to worsen.