This work examines the adsorption, desorption, and reversibility of five oxyanions (chloride, nitrate, sulfate, phosphate and arsenate) with ferrihydrite (Fh) using mostly thermodynamics data acquired from flow microcalorimetry. In addition, the role of aluminum incorporation on the physical and chemical properties of Fh is investigated. The adsorption of phosphate, sulfate, arsenate, chloride, and nitrate by Fh was examined at pH 5.6, and the enthalpy of interaction (DH) was found to follow a positive Hofmeister series such as ( = < < < . The DH values and reversibility of adsorption mirrored their modes of surface complexation. The adsorption/desorption and reversibility of sulfate on three Fh samples precipitated with 0, 12 and 24 mol % Al were examined at both pH 5.6 and 3.0. Results show an increase in the heat of exchange, irreversibility, and a fraction of inner-sphere complexes with increasing Al concentration and decreasing pH values.