Chemical processes occurring at the mineral-water interface exert a profound influence on the composition, behavior, and properties of smectite clay minerals used as barrier materials for repositories of high-level radioactive waste. Perhaps the most important process derives from the corrosion of steel confinement canisters by which Fe(0) is oxidized to Fe(II) by the reduction of viscinal H₂O to H₂. The resulting species namely, Fe⁰, Fe(II), and H₂, may react with structural Fe(III) in smectite or (oxyhydr)oxides, reducing Fe(III) to Fe(II) or dissolving the Fe under such conditions. Changes in the oxidation state of Fe in the crystal structures of clay minerals greatly alter many surface-sensitive properties of clays that are vitally important to the long-term stability of such barriers. Clays intended as barrier material have yet to be characterized with respect to their behavior under changing redox conditions. In the present study, we employed variable-temperature Mössbauer spectroscopy to characterize the effects of redox reactions on the phases, oxidation state, coordination environment, and clustering of Fe in a smectite from India (Kutch region) that has been proposed for use in engineered barriers. Results revealed that the original clay contained about 6% total Fe, distributed about equally between a surface-adsorbed oxide phase and octahedral Fe(III) in the smectite structure. Room-temperature Mössbauer spectra revealed a six-line pattern consistent with two populations of maghemite, which increased in intensity with decreasing temperature to 4 K. Reduction of the clay with citrate-bicarbonate-dithionite (CBD) removed the oxide phases and reduced the structural Fe(III) in the smectite to Fe(II). Spectra of the reduced-reoxidized clay revealed that the Fe in the smectite was restored to Fe(III), whereas the oxide phases were eliminated. Redox cycles in barriers, therefore, will likely alter the barrier properties.