Redox reactions of structural Fe affect many surface and colloidal properties of Fe-containing smectites in natural environments and many industrial systems, but few studies have examined the clay-water interface under oxidizing and reducing conditions. Infrared (FTIR) spectroscopy was used to investigate the effects of structural Fe oxidation state and hydration on layer Si-O stretching vibrations in Na-nontronite. Aqueous gels of unaltered, reduced, and reoxidized smectites were equilibrated at different swelling pressures, Pi, and water contents, m(w)/m(c), using a miniature pressure-membrane apparatus. One part of each gel was used for the gravimetric determination of m(w)/m(c); the other was transferred to an attenuated total reflectance cell in the FTIR spectrometer, where the spectrum of the gel was measured. The frequencies of four component peaks of Si-O stretching, nu(Si-O) in nontronite layers and of the H-O-H bending, vad-H, in the interlayer water were determined by using a curve-fitting technique. Reduction of structural Fe shifted the Si-O vibration to lower frequency and desensitized the Si-O vibration to the hydration state. A linear relation was found between nu(Si-O) and nu(H-O-H) for nontronite in each of its various oxidation states. These observations were interpreted to mean that structural Fe oxidation state has a significant impact on interfacial processes of the aqueous colloid system of Fe-rich phyllosilicates.