The use of constructed wetlands for amelioration of acid mine drainage has become a popular alternative to conventional treatment methods, but metal attenuation processes of these systems are poorly understood. Mineral phases precipitating in abiotic-aerobic and biotic-anaerobic zones of a staged constructed wetland treating a high metal load effluent with average Fe levels of about 800 mg/L Fe, pH 3.0, and acidity > 2200 mg/L were characterized by chemical dissolution, x-ray diffraction, thermal , and scanning electron microscopy analyses. Soluble species were related to these precipitates by geochemical modeling to elucidate mineral controls of the solution chemistry. Minerals precipitating in abiotic-aerobic treatment stages included ferrihydrite, lepidocrocite, goethite, jarosite, and high Fe/S ratio Fe-oxyhydroxysulfates. Within subsequent biotic-anaerobic wetland cells, Fe-oxyhydroxysulfate minerals with lower Fe/S ratios and poorly crystalline ferrihydrite prevailed, while the formation of jarosite and goethite was inhibited. Mineralogical characterizations and geochemical speciation suggested that the Fe chemistry is controlled primarily by the solubility of relatively crystalline Fe-oxyhydroxides in aerobic-abiotic zones and S-enriched Fe-oxyhydroxysulfates in anaerobic-biotic stages. The formation of gypsum, rhodochrosite, and siderite as byproducts of alkalinity-generation from dissolution of the limestone substsrate also appeared to have some impact on Fe, S, and Mn solubility controls.