Mullite is a promising material for advanced ceramic applications. The synthesis of mullite from oxides requires very high temperatures (T > 1000 °C). Here highly crystalline mullite whiskers with an average length and diameter of 2.37 ± 1.7 μm and 0.18 ± 0.11 μm, respectively, were synthesized by a fluoride-assisted method from aluminium sulfate, aluminium fluoride and fumed silica at a temperature as low as 800 °C.