Na- and Ca-exchanged smectite in aqueous suspension have been studied using the polarized attenuated total reflectance (ATR)-FTIR technique. These spectra correspond to the first reported use of polarization methods to study smectite particles in aqueous suspension. Similar to the behavior of hydrous metal oxides, a thin, stable layer of smectite particles was found to coat the ZnSe internal reflection element(IRE) upon exposure to a dilute aqueous suspension (solids concentration of 10 g dm(-3)). The A(s) spectrum was very similar to the transmission IR spectrum of the smectite indicating that the ATR-FTIR spectra are composed of a thin film of smectite in contact with a: thick layer of water. A, and Ap polarized in situ ATR-FTIR spectra of wet and dry Na- and Ca-SWy-1 indicated that the smectite particles were highly oriented with the (001) face parallel to the surface of the ZnSe: IRE. The A, polarized ATR-FTIR spectra clearly resolved the position of the perpendicular Si-O stretching vibration at 1075 cm(-1) for Ca-SWy-1 and at 1084 cm(-1) for Na-SWy-1. The thickness of the smectite deposit was estimated using a modified Beer's law plot obtained using transmission IR methods. The absorption coefficient for the Si-O band was 3.6 x 10(4) cm(-1), or 1.5 mAU per fundamental layer (0.96 nm) of smectite. On the basis of these values, the thickness of the smectite layer coating the ZnSe IRE was estimated to be in the range of 25-50 nm.