Infrared spectroscopy was used to investigate how nu(Si-O), the frequency of Si-O stretching, in layers of Li- and Na-montmorillonite and nu(2), the frequency of H-O-H bending, in the interlayer water were affected by increasing the concentration, C, of LiCl and NaCl, respectively. Oriented gels of Li- and Na-montmorillonite equilibrated with solutions having various values of 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 water content. 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 results showed that increasing C had little or no effect on the relation between nu(Si-O) and m(w)/m(c), or on the relation between nu(2) and m(w)/m(c). The vibrational coupling of Si-O stretching in the montmorillonite layers and H-O-H bending in the interlayer water was demonstrated. It was essentially independent of the species of exchangeable cation and C. Thus neither the species of exchangeable cation nor C had any specific effect on the structure of the clay layers or the structure of the interlayer water.