The reduction of structural Fe affects many surface chemical properties of smectite clay minerals, including their hydration and interfacial forces. To expand our knowledge of these processes, Fourier transform infrared (FTIR) spectroscopy was used to investigate the effects of structural Fe oxidation state and hydration on layer Si-O stretching vibrations in montmorillonite. Aqueous gels of unaltered, reduced, and reoxidized smectites were equilibrated at different water contents, m(w)/m(c), and swelling pressures, Pi, using a miniature pressure membrane apparatus. One part of each gel was used for the gravimetric determination of m(w)/m(c). The other part 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, v(Si-O), in montmorillonite layers and of the H-O-H bending, v(2), in the interlayer water were determined by using a curve-fitting technique. Reduction of structural Fe shifted the out-of-plane Si-O vibration to higher frequency and the in-plane Si-O vibration to lower frequency. A linear relation was found between v(Si-O) and v(2) for each oxidation state of the montmorillonite. These observations were interpreted tu mean that the structural Fe oxidation state is a primary factor in the control of interfacial interactions and processes of the aqueous clay colloid system.