Sulfur has played a major role in the formation and alteration of outcrops, rocks, and soils at the Mars Exploration Rover landing sites on Meridiani Planum and in Gusev crater. Jarosite, hematite, and evaporite sulfates (e.g., Mg sulfates) occur along with siliciclastic sediments in outcrops at Meridiani Planum. Outcrops and rocks on the West Spur of the Columbia Hills in Gusev crater appear to be extensively altered as suggested by their relative “softness” as compared to crater floor basalts, high Fe³⁺/Fe_(total), iron mineralogy dominated by nanophase Fe³⁺ oxides, hematite and/or goethite, and high Br, S, and Cl concentrations beneath outcrop surfaces. Several hypotheses have been suggested for the aqueous formation of sulfate-bearing phases on the surface of Mars including (1) the oxidative weathering of ultramafic igneous rocks containing sulfides; (2) sulfuric acid weathering of basaltic materials by solutions enriched by volcanic gases (e.g., SO₂); and (3) acid fog (i.e., vapors rich in H₂SO₄) weathering of basaltic or basaltic-derived materials. All three processes invoke acid-sulfate weathering environments.