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SF6 is generally treated as thermally stable and inert for applications below 500 °C. This work investigates the thermal stability of pure SF6 gas under 1.2 atm pressure between 200 and 450 °C in the presence of construction metals (Cu, Al), without any applied electric field. The obtained experimental results indicate that SF6 may react with metallic surfaces forming solid and gaseous by-products, either in the gas matrix or diffused in the metallic surfaces. The phenomenon is enhanced in the presence of adsorbed moisture. For copper surfaces, sulfide layers are formed. By-products are not formed for pure Al surfaces. However, when Al is covered by a few micrometres thick Al2O3 film, hot SF6 molecules have a structure change effect, i.e. reduce porosity in the oxide and in the substrate, provide smooth transition layers Al/Al2O3 and increase the Al2O3 layer width. In the presence of moisture this phenomenon is significantly intensified and a diffused overlayer of AlF3 also forms. The by-products in the gas matrix are mainly sulfur oxides for hot spot temperatures below 300 °C, while at higher temperatures oxyfluorides SO2Fx and HF are mainly formed. These by-products are either toxic or corrosive. Thus, the thermal stability issue of SF6 may have to be reconsidered.