The origin of alunite and natroalunite from a kaolin deposit (Equivocada Mine, Rio Negro Province) has been studied. Chemical analysis showed that alunite is K_0.89 and natroalunite Na_0.97, in agreement with X ray diffraction data. The high contents of minor elements in alunite, especially Ba and Sr are compatible with a hydrothermal origin in contrast to natroalunite whose very low values suggest a secondary origin. Alunite has a δ^{34S = +15.9‰ and a δ18O = +12.9‰, while in natroalunite these values are -5.8‰ and +19.0‰ respectively. δ34S values suggest that alunite formed from a hypogenic process of hydrothermal origin whereas natroalunite is probably related to a supergen origin. The occurrence of alunite in veins crosscutting the other minerals, indicates that it was the last mineral to crystallize.}