Experimental procedures in this report summarize attempts to synthesize potassium-, hydronium-, sodium-, and mixed-composition (hydronium-bearing) jarosites. After experimentation, some acceptable combinations of chemical and physical factors were found to routinely synthesize chemically different jarosites, which were used as part of a characterization study of some natural and synthetic jarosites. Jarosite is a sulfate mineral that is common in geologic settings where iron sulfide minerals such as pyrite or marcasite are subject to oxidative weathering. The presence of jarosite is a hallmark of acidic conditions. Jarosite forms as a secondary mineral in weathered sulfidic ores, coal mine wastes, on oxidizing mine wastes, and in streams affected by acid mine drainage. Jarosite also forms in acid-sulfate soils, as an alteration product of sulfidic shales, and in hydrothermal environments. Because of its important role in earth surface processes, the USGS is conducting mineralogical, geochemical, and remote sensing studies to characterize natural jarosites and to elucidate its stability range and conditions of formation. The presence of jarosite is not limited to the Earth. The recent identification of jarosite in the rocks at the Meridiani Planum on Mars has given planetary geologists reason to think that jarosite may be evidence of relict lacustrine (lake systems) or hydrothermal (water enriched magma emanations) systems on the Martian surface. USGS scientists are currently developing jarosite detection systems that can be placed on orbiting spectrometers. One day these systems may guide Mars landers to sites abundant in jarosite, sites most likely to have sheltered ancient Martian life.