The changes in the H-O-H stretching vibration in the interlayer water and Si-O stretching vibration of a Na-montmorillonite (MMT) structure in the MR (Fourier transform infrared) spectra provide insight into the effect of fluids of different dielectric Constants on the clay structure. Mechanisms by which the different fluids of varying polarities enter into the clay interlayer and the rates at which these molecules interact with the clay structure and the interlayer water are Studied at the molecular level using six different fluids with dielectric constants ranging from 110 to 2.4. The shift in H-O-H bending vibrations of interlayer water and changes in the Si-O vibration bands of MMT occur almost immediately after mixing with the solvent regardless of the polarity of the solvent. However, the extent and the rate of changes in H-O-H bending and Si-O stretching are dependent on the polarity of the solvent. Results show a very good correlation between the polarity of the solvent and the shift in H-O-H bending of interlayer water, and also between the polarity of the fluids and the d((001)) spacing of the MMT-solvent samples. Low polar fluids such as methanol tend to make weak electrostatic interactions with clay surface oxygen and interlayer-water molecules, which result in an increase in interlayer spacing. Although, the alteration of the Si-O structure due to high polar molecules such as formamide is a continuous process, the influence of nonpolar fluids such as TCE (trichloroethylene) on the Si-O structure is almost instantaneous, which may result in high hydraulic conductivity in the clay. (C) 2009 Elsevier Inc. All rights reserved.