Abstract: Infrared spectra (4000–1200 cm−1) were obtained for several homoionic montmorillonite films on which tetramethylene sulfoxide (TMSO) or thiolane were adsorbed at various temperatures and for different periods of exposure. The spectra indicate that the Na-, H-, and natural montmorillonite complexes contain a physically adsorbed species, whereas transition metal-montmorillonite complexes contain both physically adsorbed and metal-complexed species in their interlamellar spaces. Apparently, thiolane adsorbed on most montmorillonites undergoes oxidation to TMSO in an air atmosphere. Consistent with the mechanism proposed earlier for aqueous solutions, the rate of sulfoxide formation increases by increasing the pH of the suspensions from which the clay films were deposited or by increasing the concentration of the water molecules in the interlamellar spaces. The infrared spectra of γ-thiolactone adsorbed on Co-montmorillonites suggest that sulfoxide-type molecules are formed which chelate to the Co ions in the interlamellar spaces.