In the present study, we investigated the behavior of arsenic (As) during the transformation of poorly crystalline iron(III) hydroxide, ferrihydrite. Adsorbed As on solids gradually decreased as ferrihydrite transformed to crystalline goethite and hematite. X-ray absorption fine structure (XAFS) analysis indicated that the As immobilization dominantly resulted from the incorporation of arsenate into poorly crystalline ferrihydrite under present experimental conditions.