Alumina was leached from high-alumina coal fly ash with a NH4HSO4 + H2SO4 mixed solution. The leaching behavior was systematically investigated, as were factors such as mixed solution composition and concentration, reaction time and temperature, and liquid-to-solid ratio. In optimal conditions, the alumina extraction ratios can reach 87.8%, 91.1%, and 87.5% for the three different sources of coal fly ash, respectively. The phases and morphology of the raw materials and residues were also analyzed through X-ray diffraction and scanning electron microscopy energy dispersive spectroscopy. Results showed that mullite, which is the main form taken by alumina in high-alumina coal fly ash, can be effectively decomposed to release the alumina and induce a new amorphous silica phase in mixed solution. (C) 2015 Elsevier B.V. All rights reserved.