The frequencies of Si-O stretching, nu(Si-O), in layers of montmorillonite and of H-O-H bending, nu(2), in the interlayer water were investigated by infrared spectroscopy. Oriented gels of Li- and Na-montmorillonite equilibrated at different water contents, m(w)/m(c), were obtained by 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 FTIR spectrometer where the spectrum of the gel was measured by attenuated total reflectance. Thus, the dependence of both nu(Si-O) and nu(2) on m(w)/m(c) was determined in the same samples. The absorption bands for Si-O stretching were resolved by using a curve-fitting technique to determine the frequency, area, and height of the component peaks. Two prominent peaks, labeled peak III and peak IV, were selected for analysis. It was found that nu(Si-O) for each peak and the ratio of the areas (or heights) of the two peaks were all exponential functions of m(c)/m(w). Moreover, a linear relation was found between ln(nu(Si-O)) and ln(nu(2)). These results were interpreted to mean that the Si-O stretching vibrations in the montmorillonite layers and the H-O-H bending vibrations in the interlayer water are coupled and that both change as the layers come closer together.