Jarosite [KFe₃(OH)₆(SO₄)₂], a pale yellow mineral, is a common product of the oxidation of iron sulfides in acidic environments, such as sulfuric soil horizons and acid mine drainage. Jarosite has recently attracted much attention because it has also been identified in the evaporitic deposits of Mars. Here, we report the hydrolisis products of synthetic K-jarosite at different pH, temperature, and phosphate and salt concentrations by X ray diffraction, diffuse reflectance spectroscopy, scanning and transmission electron microscopy, and chemical analysis. We synthesized jarosite by first dissolving 0.1 mol of Fe₂(SO₄)₃, 0.4 mol MgSO₄, 0.04 mol of AlCl₃, 0.06 mol of Na₂SO₄, 0.002 mol of CaCl₂, 0.004 mol of MnSO₄ and variable amounts of KH₂(PO₄) in 160 mL of de-ionized water. A concentrated solution of KOH was then added to reach a pH of 2 and a volume of 220 mL before shaking the resulting suspension at room temperature for two weeks. At this time, the solids in the suspension consisted exclusively of jarosite. In a factorial experiment, portions of the suspension were then mixed with saline solutions of the same ionic composition as the one used in the synthesis [except for Fe(III) and PO₄] and water at different proportions so that the resulting electrical electrical conductivity (EC) of the suspension (first factor) was either 60, 20 or 1 mS/cm. The pH (second factor) was raised to 4, 6 or 8. Finally, the suspensions were aged for 6 months at a temperature (third factor) of either 303 or 333 K. During aging, pH and salt concentration were periodically adjusted to the target values. High salt concentration inhibited the transformation of jarosite except at pH 8 (Figure 1), suggesting that jarosite is stable even at circum-neutral pH if water activity is sufficiently low. Nanophase hematite was the most common product of the transformation of jarosite, particularly when salt and phosphate concentrations were high. Nanogoethite was formed only when salt concentration was low (i.e., at high water activity) and pH was 6, and was never an intermediate phase in the transformation from jarosite into hematite. At pH 8 jarosite was converted fastly (<7 days) to 2-line ferrihydrite than evolved, with time, to hematite or to a mixture of 2-line ferrihydrite and nano-lepidocrocite.