Jarosite minerals, i.e. iron (III) sulfates with the monovalent cations potassium, sodium and hydronium, have been identified at 13 localities evenly spread over Sweden. The end members jarosite (potassium), natrojarosite (sodium) and hydronium jarosite constitute an isomorphous solid solution series, with rather regular changes of cell-axes. A closer inspection of the cell-axes by refined X-ray diffraction methods reveals that the potassic variety dominates in the material examined. However, sodium and hydronium-dominant minerals have each been found in one sample. Jarosite minerals are formed under acid conditions with high redox potential. Suitable environments are sulfide mine dumps, where jarosites seem to be almost ubiquitous in small amounts.