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In the past, several contributions dealing with the effects of the exchange cations and the hydration status on infrared (IR) absorptions of water molecules in phyllosilicates have been published (e.g., Fripiat et al., 1960;Farmer and Russell, 1971), although spectral resolution was hindered by the overlapping of many intense bands. The investigation of the near infrared (NIR) spectra of Ca-bentonites has enabled us to distinguish the absorptions due to water molecules from those of structural hydroxyl groups and to perform the spectral assignments of the overtone and combination bands observed in phyllosilicates (Cariati et al., 1981). NIR spectra allow easier detection of the several types of water molecules simultaneously present in the interlayer. Thus we have made a comparative study on some M(I)-and M(II)-bentonites (M(I) = Li, Na, K; M(II) = Ca, Ba, Ni, Cu). The aim of this investigation is: (1) to seek confirmation of previous IR results; (2) to correlate changes in the vibrational properties of distinguishable water molecules with the nature of the exchange cations and the hydration status of the samples. EXPERIMENTALNatural bentonite from Uri, Sardinia, was used in this study. Pietracaprina et al. (1972) reported a structural formula for this material of (Sir.878A10az2)(Ala.l~6Fe(III)0.~a~Mg0.560(OH)40~0 and a cation-exchange capacity (CEC) of 65 meq/100 g. Homoionic samples were obtained by usual exchange procedures (Bart et al., 1979) using metal chloride solutions. The exchanged samples were equilibrated over H2SO4/H20 solutions or dehydrated under vacuum to obtain different water/ion ratios. The final water content was monitored by thermogravimetric analysis using a Perkin-Elmer TGS-2 apparatus under a nitrogen stream and a heating rate of 5~ NIR spectra were recorded with a Beckman Acta M IV spectrophotometer equipped with a reflectance unit. BaSt4 was the reference sample, The spectra were fitted to a sum of Gaussian-shaped bands in the 1360-2100-nm range using a least-squares program run on an Univac 1108 computer. The quality of the fit was judged from the agreement factor which indicated that the absorptions were reasonably described, as a first approximation, by Gaussian shape. It should be noted that the hydroxyl-group bands had almost the identical half-widths and positions in all samples, whereas the water absorptions were markedly different with varying conditions. RESULTSTypical spectra of bentonites (that of the Ca-exchanged sample is shown in Figure 1), show in the NIR region bands at 1409 and 2205 nm due to framework hydroxyl groups plus three main absorptions attributable to water molecules. Shoulders A and C (Figure 1) are due to water-OH groups engaged in strong hydrogen bonds (Cariati et al., 1981) and band B (Figure 1) is due to "free" water molecules involved in weak interactions with the low-basicity oxygen ions of the silicate surfaces (Cariati et al., 1981). Assignments of the NIR absorptions are listed in Table 1.Spectral changes appeared by varying the exchange cation or de...