The thermal decomposition of Saphane alunite from Kutahya, Turkey, has been studied using ambient- and high-temperature X-ray diffraction analysis, differential thermal analysis, and thermogravimetric analysis. The alunite content in the ore was about 47.5%. The first endothermic peak at about 545 degreesC is due to the dehydration of the structural water and the transformation of alunite into KAl(SO4)(2) and amorphous Al2O3. The small exothermic peak at about 712 degreesC is probably caused by the recrystallization of KAl(SO4)(2) into K3Al(SO4)(3) and Al-2(SO4)(3). So far, no work has reported this new phase, K3Al(SO4)(3), resulting from the exothermic reaction of alunite. Moreover, the d and I values of unidentified peaks in the previous literature gave reflections that were surprisingly identical to those of the K3Al(SO4)(3) peaks. The second endothermic peak at about 810 degreesC is due to the desulfurization by which K3Al(SO4)(3) and amorphous Al-2(SO4)(3) decompose into K2SO4 (arcanite) and two polymorphic varieties of Al2O3 (kappa and gamma). Above 800 degreesC, only gamma-alumina and arcanite were observed.